mission: explicitly set backtransition mission item altitude

Signed-off-by: RomanBapst <bapstroman@gmail.com>

src/modules/navigator/mission.cpp

@@ -852,6 +852,7 @@ Mission::set_mission_items()
 				    && !_navigator->get_land_detected()->landed) {
 
 					set_vtol_transition_item(&_mission_item, vtol_vehicle_status_s::VEHICLE_VTOL_STATE_MC);
+					_mission_item.altitude = _navigator->get_global_position()->alt;
 
 					new_work_item_type = WORK_ITEM_TYPE_MOVE_TO_LAND_AFTER_TRANSITION;
 				}