flight_mode_manager: invalidate dist to bottom/ground on terrain reset

 - this allows the terrain hold to simply reset on the next valid update
   rather than trying to adjust the setpoints

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

@@ -66,6 +66,8 @@ void FlightTask::_checkEkfResetCounters()
 	if (_sub_vehicle_local_position.get().dist_bottom_reset_counter != _reset_counters.hagl) {
 		_ekfResetHandlerHagl(_sub_vehicle_local_position.get().delta_dist_bottom);
 		_reset_counters.hagl = _sub_vehicle_local_position.get().dist_bottom_reset_counter;
+
+		_dist_to_bottom = NAN;
 	}
 
 	if (_sub_vehicle_local_position.get().vz_reset_counter != _reset_counters.vz) {

src/modules/flight_mode_manager/tasks/ManualAltitude/FlightTaskManualAltitude.cpp

@@ -279,6 +279,11 @@ void FlightTaskManualAltitude::_ekfResetHandlerHeading(float delta_psi)
 	_stick_yaw.ekfResetHandler(delta_psi);
 }
 
+void FlightTaskManualAltitude::_ekfResetHandlerHagl(float delta_hagl)
+{
+	_dist_to_ground_lock = NAN;
+}
+
 void FlightTaskManualAltitude::_updateSetpoints()
 {
 	_stick_yaw.generateYawSetpoint(_yawspeed_setpoint, _yaw_setpoint, _sticks.getYawExpo(), _yaw, _deltatime, _unaided_yaw);

src/modules/flight_mode_manager/tasks/ManualAltitude/FlightTaskManualAltitude.hpp

@@ -56,6 +56,8 @@ class FlightTaskManualAltitude : public FlightTask
 
 protected:
 	void _ekfResetHandlerHeading(float delta_psi) override; /**< adjust heading setpoint in case of EKF reset event */
+	void _ekfResetHandlerHagl(float delta_hagl) override;
+
 	virtual void _updateSetpoints(); /**< updates all setpoints */
 	virtual void _scaleSticks(); /**< scales sticks to velocity in z */
 	bool _checkTakeoff() override;