Fix mismatched allocation with empty string keys and payloads

Producer::NodeProduce() creates a temporary buffer backed by
a `new char[0]` for keys and payloads that were empty buffers, since
node::Buffer::Data() returns NULL for these.

The [NAN docs](https://github.com/nodejs/nan/blob/main/doc/buffers.md#nannewbuffer)
indicate that the data managed by this buffer will be freed by free() in
the absence of a custom free callback, which leads ASAN to complain:
```
    =================================================================
==28555==ERROR: AddressSanitizer: alloc-dealloc-mismatch (operator new [] vs free) on 0x60200010fc70

    #0 0x7f6c0e98fb6f in __interceptor_free ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:123
    Blizzard#1 0x127e3d4 in v8::internal::BackingStore::~BackingStore() (/usr/bin/node+0x127e3d4)
    Blizzard#2 0xc0934a in std::_Sp_counted_deleter<v8::BackingStore*, std::default_delete<v8::BackingStore>, std::allocator<void>, (__gnu_cxx::_Lock_policy)2>::_M_dispose() (/usr/bin/node+0xc0934a)
    Blizzard#3 0x10a9fce in v8::internal::ArrayBufferSweeper::SweepingJob::SweepYoung() (/usr/bin/node+0x10a9fce)
    Blizzard#4 0x10aab2e in std::_Function_handler<void (), v8::internal::ArrayBufferSweeper::RequestSweep(v8::internal::ArrayBufferSweeper::SweepingType, v8::internal::ArrayBufferSweeper::TreatAllYoungAsPromoted)::{lambda()Blizzard#1}>::_M_invoke(std::_Any_data const&) (/usr/bin/node+0x10aab2e)
    Blizzard#5 0xd49370 in node::(anonymous namespace)::PlatformWorkerThread(void*) (/usr/bin/node+0xd49370)
    Blizzard#6 0x7f6c0e591ea6 in start_thread (/lib/x86_64-linux-gnu/libpthread.so.0+0x7ea6)
    Blizzard#7 0x7f6c0e4afa6e in __clone (/lib/x86_64-linux-gnu/libc.so.6+0xfba6e)

0x60200010fc70 is located 0 bytes inside of 1-byte region [0x60200010fc70,0x60200010fc71)
allocated by thread T0 here:
    #0 0x7f6c0e9917a7 in operator new[](unsigned long) ../../../../src/libsanitizer/asan/asan_new_delete.cpp:102
    Blizzard#1 0x7f6963812e97 in NodeKafka::Producer::NodeProduce(Nan::FunctionCallbackInfo<v8::Value> const&) ../src/producer.cc:501
    Blizzard#2 0x7f69637f18f7 in FunctionCallbackWrapper ../node_modules/nan/nan_callbacks_12_inl.h:177
    Blizzard#3 0x7f6c0430d545  (/usr/bin/node+0x14e5545)
    Blizzard#4 0x7f6be45470df  (<unknown module>)
    Blizzard#5 0x18e3d1b in Builtins_InterpreterEntryTrampoline (/usr/bin/node+0x18e3d1b)
    Blizzard#6 0x7f6be453ef57  (<unknown module>)
    Blizzard#7 0x7f6be454d668  (<unknown module>)
    Blizzard#8 0x18e3d1b in Builtins_InterpreterEntryTrampoline (/usr/bin/node+0x18e3d1b)
    Blizzard#9 0x18e3d1b in Builtins_InterpreterEntryTrampoline (/usr/bin/node+0x18e3d1b)
    Blizzard#10 0x7f6be453ce66  (<unknown module>)
    Blizzard#11 0x7f6be44fdf61  (<unknown module>)
    Blizzard#12 0x7f6be4543bee  (<unknown module>)
    Blizzard#13 0x18e3d1b in Builtins_InterpreterEntryTrampoline (/usr/bin/node+0x18e3d1b)
    Blizzard#14 0x7f6be44fddea  (<unknown module>)
    Blizzard#15 0x18e3d1b in Builtins_InterpreterEntryTrampoline (/usr/bin/node+0x18e3d1b)
    Blizzard#16 0x18e3d1b in Builtins_InterpreterEntryTrampoline (/usr/bin/node+0x18e3d1b)
    Blizzard#17 0x18e3d1b in Builtins_InterpreterEntryTrampoline (/usr/bin/node+0x18e3d1b)
    Blizzard#18 0x18e3d1b in Builtins_InterpreterEntryTrampoline (/usr/bin/node+0x18e3d1b)
    Blizzard#19 0x18e3d1b in Builtins_InterpreterEntryTrampoline (/usr/bin/node+0x18e3d1b)
    Blizzard#20 0x18e20db in Builtins_JSEntryTrampoline (/usr/bin/node+0x18e20db)
    Blizzard#21 0x18e1e02 in Builtins_JSEntry (/usr/bin/node+0x18e1e02)
    Blizzard#22 0x105f17a in v8::internal::(anonymous namespace)::Invoke(v8::internal::Isolate*, v8::internal::(anonymous namespace)::InvokeParams const&) (/usr/bin/node+0x105f17a)
    Blizzard#23 0x1060213 in v8::internal::Execution::Call(v8::internal::Isolate*, v8::internal::Handle<v8::internal::Object>, v8::internal::Handle<v8::internal::Object>, int, v8::internal::Handle<v8::internal::Object>*) (/usr/bin/node+0x1060213)
    Blizzard#24 0xf22a7c in v8::Function::Call(v8::Local<v8::Context>, v8::Local<v8::Value>, int, v8::Local<v8::Value>*) (/usr/bin/node+0xf22a7c)
    Blizzard#25 0xbc911a in node::InternalMakeCallback(node::Environment*, v8::Local<v8::Object>, v8::Local<v8::Object>, v8::Local<v8::Function>, int, v8::Local<v8::Value>*, node::async_context) (/usr/bin/node+0xbc911a)
    Blizzard#26 0xbc925d in node::MakeCallback(v8::Isolate*, v8::Local<v8::Object>, v8::Local<v8::Function>, int, v8::Local<v8::Value>*, node::async_context) (/usr/bin/node+0xbc925d)
    Blizzard#27 0x7f6963835c25 in Nan::AsyncResource::runInAsyncScope(v8::Local<v8::Object>, v8::Local<v8::Function>, int, v8::Local<v8::Value>*) ../node_modules/nan/nan.h:631
    Blizzard#28 0x7f6963835c25 in Nan::Callback::Call_(v8::Isolate*, v8::Local<v8::Object>, int, v8::Local<v8::Value>*, Nan::AsyncResource*) const ../node_modules/nan/nan.h:1881
    Blizzard#29 0x7f6963835c25 in Nan::Callback::Call(int, v8::Local<v8::Value>*) const ../node_modules/nan/nan.h:1825
    Blizzard#30 0x7f6963835c25 in NodeKafka::Workers::ConnectionMetadata::HandleOKCallback() ../src/workers.cc:116
    Blizzard#31 0x7f69637b6bac in Nan::AsyncWorker::WorkComplete() ../node_modules/nan/nan.h:2008
    Blizzard#32 0x7f69637b6bac in Nan::AsyncExecuteComplete(uv_work_s*) ../node_modules/nan/nan.h:2365
    Blizzard#33 0x7f69637b6bac in Nan::AsyncExecuteComplete(uv_work_s*, int) ../node_modules/nan/nan.h:2369
    Blizzard#34 0x18bb75c in uv__work_done ../deps/uv/src/threadpool.c:329

Thread T4 created by T0 here:
    #0 0x7f6c0e93b2a2 in __interceptor_pthread_create ../../../../src/libsanitizer/asan/asan_interceptors.cpp:214
    Blizzard#1 0x18cdf7b in uv_thread_create_ex ../deps/uv/src/unix/thread.c:172
    Blizzard#2 0x18cdf7b in uv_thread_create ../deps/uv/src/unix/thread.c:126
    Blizzard#3 0xd4c115 in node::WorkerThreadsTaskRunner::WorkerThreadsTaskRunner(int) (/usr/bin/node+0xd4c115)
    Blizzard#4 0xd4c3cb in node::NodePlatform::NodePlatform(int, v8::TracingController*, v8::PageAllocator*) (/usr/bin/node+0xd4c3cb)
    Blizzard#5 0xc6f785 in node::InitializeOncePerProcessInternal(std::vector<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > const&, node::ProcessInitializationFlags::Flags) (/usr/bin/node+0xc6f785)
    Blizzard#6 0xc70c83 in node::Start(int, char**) (/usr/bin/node+0xc70c83)
    Blizzard#7 0x7f6c0e3d7d09 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x23d09)

SUMMARY: AddressSanitizer: alloc-dealloc-mismatch ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:123 in __interceptor_free
```

Reproducer:
```js
const { Producer } = require('.');
var producer = new Producer({
  'client.id': 'kafka-test',
  'metadata.broker.list': '127.0.0.1:9092',
  'dr_cb': true,
  'debug': 'all'
});
producer.connect({}, function(err) {
  if (err) {
    console.error('Error connecting to Kafka:', err);
    return;
  }
  producer.produce('test', null, Buffer.from(''), '');

  producer.disconnect();
});
```

There should not be a need to allocate a buffer in this scenario.
Use a single static character array holding a null-terminator as the
payload/key instead.

Author: mszabo-wikia

src/producer.cc

@@ -7,6 +7,7 @@
  * of the MIT license.  See the LICENSE.txt file for details.
  */
 
+#include <array>
 #include <string>
 #include <vector>
 
@@ -474,6 +475,7 @@ NAN_METHOD(Producer::NodeProduce) {
 
   size_t message_buffer_length;
   void* message_buffer_data;
+  static std::array<char, 1> empty_string{'\0'};
 
   if (info[2]->IsNull()) {
     // This is okay for whatever reason
@@ -497,9 +499,8 @@ NAN_METHOD(Producer::NodeProduce) {
     message_buffer_data = node::Buffer::Data(message_buffer_object);
     if (message_buffer_data == NULL) {
       // empty string message buffer should not end up as null message
-      v8::Local<v8::Object> message_buffer_object_emptystring = Nan::NewBuffer(new char[0], 0).ToLocalChecked();
-      message_buffer_length = node::Buffer::Length(message_buffer_object_emptystring);
-      message_buffer_data = node::Buffer::Data(message_buffer_object_emptystring);
+      message_buffer_length = 0;
+      message_buffer_data = reinterpret_cast<void*>(empty_string.data());
     }
   }
 
@@ -527,9 +528,8 @@ NAN_METHOD(Producer::NodeProduce) {
     key_buffer_data = node::Buffer::Data(key_buffer_object);
     if (key_buffer_data == NULL) {
       // empty string key buffer should not end up as null key
-        v8::Local<v8::Object> key_buffer_object_emptystring = Nan::NewBuffer(new char[0], 0).ToLocalChecked();
-        key_buffer_length = node::Buffer::Length(key_buffer_object_emptystring);
-        key_buffer_data = node::Buffer::Data(key_buffer_object_emptystring);
+        key_buffer_length = 0;
+        key_buffer_data = reinterpret_cast<void*>(empty_string.data());
     }
   } else {
     // If it was a string just use the utf8 value.