precland: set the previous position setpoint to invalid, in the position setpoint triplet

src/modules/navigator/precland.cpp

@@ -85,6 +85,7 @@ PrecLand::on_activation()
 	position_setpoint_triplet_s *pos_sp_triplet = _navigator->get_position_setpoint_triplet();
 
 	pos_sp_triplet->next.valid = false;
+	pos_sp_triplet->previous.valid = false;
 
 	// Check that the current position setpoint is valid, otherwise land at current position
 	if (!pos_sp_triplet->current.valid) {