message: core: add missing return after borrow
  Fixes a lockup.
diff: @@ -222,6 +222,7 @@ void MAVLinkCommands::receive_command_ack(mavlink_message_t message)
  _parent.register_timeout_handler(
  std::bind(&MAVLinkCommands::receive_timeout, this),
  work->retries_to_do * work->timeout_s, &_timeout_cookie);
  + _work_queue.return_front();
  break;
  }
  }
  