message: mission: fix off-by-one for RTL after mission
  RTL is connected to the last mission item and does not require an
  additional step in the mission progress info.
  The off-by-one error was due because we increment item_i at the end of a
  loop iteration, although that's not required in the last iteration.
diff: @@ -695,6 +695,10 @@ void MissionImpl::assemble_mavlink_messages()
  ++item_i;
  }
  + // We need to decrement the item_i again because it was increased in the loop above
  + // but the RTL item below still belongs to the last mission item.
  + --item_i;
  +
  if (_enable_return_to_launch_after_mission) {
  std::shared_ptr<mavlink_message_t> message_rtl(new mavlink_message_t());
  mavlink_msg_mission_item_int_pack(GCSClient::system_id,
  