message: [C++] Fix memory leak when using grpc
diff: @@ -276,10 +276,12 @@ template<class T> class SerializationTraits<flatbuffers::grpc::Message<T>> {
  Slice slice;
  if (!buf->TrySingleSlice(&slice).ok()) {
  if (!buf->DumpToSingleSlice(&slice).ok()) {
  + buf->Clear();
  return ::grpc::Status(::grpc::StatusCode::INTERNAL, "No payload");
  }
  }
  *msg = flatbuffers::grpc::Message<T>(slice);
  + buf->Clear();
  #if FLATBUFFERS_GRPC_DISABLE_AUTO_VERIFICATION
  return ::grpc::Status::OK;
  #else
  