message: core: fix total item count
diff: @@ -765,7 +765,8 @@ int MissionImpl::current_mission_item() const
  int MissionImpl::total_mission_items() const
  {
  std::lock_guard<std::recursive_mutex> lock(_mission_data.mutex);
  - return static_cast<int>(_mission_data.mavlink_mission_item_to_mission_item_indices.size());
  + return static_cast<int>(
  + _mission_data.mavlink_mission_item_to_mission_item_indices.rbegin()->second + 1);
  }
  void MissionImpl::subscribe_progress(Mission::progress_callback_t callback)
  