message: mission: update mavlink item map on download
  We forgot to update the mavlink mission item to mission item map which
  meant that the mission progress was unknown (-1) in case the mission is
  only downloaded but never uploaded.
diff: @@ -729,6 +729,7 @@ void MissionImpl::assemble_mission_items()
  {
  std::lock_guard<std::recursive_mutex> lock(_mission_data.mutex);
  _mission_data.mission_items.clear();
  + _mission_data.mavlink_mission_item_to_mission_item_indices.clear();
  _enable_return_to_launch_after_mission = false;
  auto new_mission_item = std::make_shared<MissionItem>();
  @@ -750,6 +751,8 @@ void MissionImpl::assemble_mission_items()
  return;
  }
  + int mavlink_item_i = 0;
  +
  for (auto &it : _mission_data.mavlink_mission_items_downloaded) {
  LogDebug() << "Assembling Message: " << int(it->seq);
  @@ -839,6 +842,11 @@ void MissionImpl::assemble_mission_items()
  result = Mission::Result::UNSUPPORTED;
  break;
  }
  +
  + _mission_data.mavlink_mission_item_to_mission_item_indices.insert(std::pair<int, int>{
  + mavlink_item_i, static_cast<int>(_mission_data.mission_items.size())});
  +
  + ++mavlink_item_i;
  }
  // Don't forget to add last mission item.
  