src: add debug CHECKs against empty handles

These checks were useful while investigating other issues;
using empty `Local<>`s can be very un-debuggable, because that
typically does not lead to assertions with debugging information
but rather crashes based on accessing invalid memory.

PR-URL: #26125
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

src/api/callback.cc

@@ -139,6 +139,11 @@ MaybeLocal<Value> InternalMakeCallback(Environment* env,
                                        Local<Value> argv[],
                                        async_context asyncContext) {
   CHECK(!recv.IsEmpty());
+#ifdef DEBUG
+  for (int i = 0; i < argc; i++)
+    CHECK(!argv[i].IsEmpty());
+#endif
+
   InternalCallbackScope scope(env, recv, asyncContext);
   if (scope.Failed()) {
     return MaybeLocal<Value>();

src/node_errors.cc

@@ -710,6 +710,7 @@ void DecorateErrorStack(Environment* env,
 void FatalException(Isolate* isolate,
                     Local<Value> error,
                     Local<Message> message) {
+  CHECK(!error.IsEmpty());
   HandleScope scope(isolate);
 
   Environment* env = Environment::GetCurrent(isolate);