message: core: fix null dereference warning
  The null dereference warning caught this one!
diff: @@ -31,7 +31,7 @@ bool Connection::start_mavlink_receiver()
  void Connection::stop_mavlink_receiver()
  {
  - if (!_mavlink_receiver) {
  + if (_mavlink_receiver) {
  uint8_t used_channel = _mavlink_receiver->get_channel();
  // Destroy receiver before giving the channel back.
  