navigator: add position setpoint reset helper with safe defaults

- use in reset_triplets()

src/modules/navigator/takeoff.cpp

@@ -128,6 +128,7 @@ Takeoff::set_takeoff_position()
 	struct position_setpoint_triplet_s *pos_sp_triplet = _navigator->get_position_setpoint_triplet();
 	mission_apply_limitation(_mission_item);
 	mission_item_to_position_setpoint(_mission_item, &pos_sp_triplet->current);
+
 	pos_sp_triplet->previous.valid = false;
 	pos_sp_triplet->current.yaw_valid = true;
 	pos_sp_triplet->next.valid = false;