FlightTask: Fix ekf2 reset race condition during task switch (#14692)

* FlightTask: Fix ekf2 reset race condition during task switch

During a loss of GPS data when using GPS as primary height source,
the height is reset to baro and the local position gets invalid at the
same time. This triggers a switch to altitude flight task and a setpoint
reset.
This combination of events had the effect to ignore the height reset,
the large sudden height error could create an abrupt change of altitude
or even a crash.
The ekf2 reset is now done at the beginning of each update call.

src/lib/flight_tasks/tasks/Failsafe/FlightTaskFailsafe.cpp

@@ -49,6 +49,8 @@ bool FlightTaskFailsafe::activate(vehicle_local_position_setpoint_s last_setpoin
 
 bool FlightTaskFailsafe::update()
 {
+	bool ret = FlightTask::update();
+
 	if (PX4_ISFINITE(_position(0)) && PX4_ISFINITE(_position(1))) {
 		// stay at current position setpoint
 		_velocity_setpoint(0) = _velocity_setpoint(1) = 0.f;
@@ -72,5 +74,5 @@ bool FlightTaskFailsafe::update()
 		_acceleration_setpoint(2) = NAN;
 	}
 
-	return true;
+	return ret;
 }