message: add assertion for null/reset connection file event
diff: @@ -386,6 +386,7 @@ void ConnectionImpl::write(Buffer::Instance& data, bool end_stream) {
  // doWriteReady into thinking the socket is connected. On macOS, the underlying write may fail
  // with a connection error if a call to write(2) occurs before the connection is completed.
  if (!connecting_) {
  + ASSERT(file_event_ != nullptr, "ConnectionImpl file event was unexpectedly reset");
  file_event_->activate(Event::FileReadyType::Write);
  }
  }
  