FlightTask: set yaw_setpoint to NAN when yaw should not be controlled

src/modules/flight_mode_manager/tasks/FlightTask/FlightTask.cpp

@@ -124,6 +124,7 @@ void FlightTask::_evaluateVehicleLocalPosition()
 
 		// yaw
 		_yaw = _sub_vehicle_local_position.get().heading;
+		_is_yaw_good_for_control = _sub_vehicle_local_position.get().heading_good_for_control;
 
 		// position
 		if (_sub_vehicle_local_position.get().xy_valid) {

src/modules/flight_mode_manager/tasks/FlightTask/FlightTask.hpp

@@ -231,6 +231,7 @@ protected:
 	matrix::Vector3f _velocity; /**< current vehicle velocity */
 
 	float _yaw{}; /**< current vehicle yaw heading */
+	bool _is_yaw_good_for_control{}; /**< true if the yaw estimate can be used for yaw control */
 	float _dist_to_bottom{}; /**< current height above ground level */
 	float _dist_to_ground{}; /**< equals _dist_to_bottom if valid, height above home otherwise */