fix: memory leak

Author: yulingtianxia

dart_native/ios/Classes/native_runtime.h

@@ -41,7 +41,7 @@ native_get_class(const char *className, Class superclass);
 /// @param queue dispatch queue for async method.
 /// @param args arguments passed to method.
 /// @param dartPort port for dart isolate.
-/// @param stringTypeBitmask first bit is for return value.
+/// @param stringTypeBitmask bitmask for checking if an argument is a string.
 DN_EXTERN void * _Nullable
 native_instance_invoke(id object, SEL selector, NSMethodSignature *signature, dispatch_queue_t queue, void * _Nonnull * _Nullable args, void (^callback)(void *), Dart_Port dartPort, int64_t stringTypeBitmask);
 

dart_native/lib/src/ios/runtime/message.dart

@@ -136,6 +136,8 @@ dynamic _msgSend(Pointer<Void> target, SEL selector,
     }
     free(typeEncodingsPtrPtr);
     return result;
+  } else {
+    free(typeEncodingsPtrPtr);
   }
 }