Navigator: Initialize RTL state properly

src/modules/navigator/rtl.cpp

@@ -99,6 +99,8 @@ RTL::on_activation()
 	set_current_position_item(&_mission_item);
 	struct position_setpoint_triplet_s *pos_sp_triplet = _navigator->get_position_setpoint_triplet();
 	mission_item_to_position_setpoint(&_mission_item, &pos_sp_triplet->current);
+	pos_sp_triplet->previous.valid = false;
+	pos_sp_triplet->next.valid = false;
 
 	/* for safety reasons don't go into RTL if landed */
 	if (_navigator->get_land_detected()->landed) {