Change plain char buffer to unique_ptr to free memory at function exit.

Visible in `rr record simple` with asan enabled rr build.


==647542==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 19 byte(s) in 1 object(s) allocated from:
    0x7fe6e70187a7 in operator new[](unsigned long) ../../../../src/libsanitizer/asan/asan_new_delete.cpp:102
    0x55a421ec5ca1 in simple_to_lower /home/bernhard/data/entwicklung/2021/rr/2021-04-25/rr/src/log.cc:62
    0x55a421ec6f3b in get_log_level /home/bernhard/data/entwicklung/2021/rr/2021-04-25/rr/src/log.cc:191
    0x55a421ec7766 in get_log_module /home/bernhard/data/entwicklung/2021/rr/2021-04-25/rr/src/log.cc:224
...

src/log.cc

@@ -59,12 +59,12 @@ static char simple_to_lower(char ch) {
 }
 
 static string simple_to_lower(const string& s) {
-  char* buf = new char[s.size() + 1];
+  std::unique_ptr<char[]> buf(new char[s.size() + 1]);
   for (size_t i = 0; i < s.size(); ++i) {
     buf[i] = simple_to_lower(s[i]);
   }
   buf[s.size()] = 0;
-  return string(buf);
+  return string(buf.get());
 }
 
 #if __has_attribute(require_constant_initialization)