message: mission: add missing busy check
diff: @@ -137,6 +137,16 @@ void MissionImpl::upload_mission_cancel()
  void MissionImpl::download_mission_async(
  const Mission::mission_items_and_result_callback_t& callback)
  {
  + if (_mission_data.last_download.lock()) {
  + _parent->call_user_callback([callback]() {
  + if (callback) {
  + std::vector<std::shared_ptr<MissionItem>> empty_items;
  + callback(Mission::Result::BUSY, empty_items);
  + }
  + });
  + return;
  + }
  +
  _parent->mission_transfer().download_items_async(
  MAV_MISSION_TYPE_MISSION,
  [this, callback](
  