message: core: fixed wrong variable
  This somehow got out of sync in the recent merges.
diff: @@ -43,7 +43,7 @@ DeviceImpl::~DeviceImpl()
  bool DeviceImpl::is_connected() const
  {
  - return _heartbeats_arriving;
  + return _connected;
  }
  void DeviceImpl::register_mavlink_message_handler(uint16_t msg_id,
  