message: mission: fix potential segfault
  I have seen a segfault happen in current_mission_item() and this is the
  only thing that potentially could have caused it, I think.
diff: @@ -820,7 +820,8 @@ int MissionImpl::current_mission_item() const
  // We want to return the current mission item and not the underlying
  // mavlink mission item.
  if (_mission_data.last_current_mavlink_mission_item >=
  - static_cast<int>(_mission_data.mavlink_mission_item_to_mission_item_indices.size())) {
  + static_cast<int>(_mission_data.mavlink_mission_item_to_mission_item_indices.size()) ||
  + _mission_data.last_current_mavlink_mission_item < 0) {
  return -1;
  }
  