message: mission: fix current mission index
  This was wrong, we should only increment the index when a new waypoint
  with a location is added.
diff: @@ -546,8 +546,6 @@ void MissionImpl::assemble_mavlink_messages()
  static_cast<int>(_mission_data.mavlink_mission_item_messages.size()),
  item_i});
  _mission_data.mavlink_mission_item_messages.push_back(message_gimbal_configure);
  -
  - ++item_i;
  }
  // The gimbal has changed, we need to add a gimbal command.
  