FlightTask: fail activation if required states are not available

src/lib/FlightTasks/tasks/FlightTaskManualAltitude.cpp

@@ -47,6 +47,9 @@ bool FlightTaskManualAltitude::activate()
 	_position_setpoint(2) = _position(2);
 	_velocity_setpoint(2) = 0.0f;
 	_setDefaultConstraints();
+
+	// altitude-mode requires to have a valid position and velocity state in D-direction
+	ret = ret &&  PX4_ISFINITE(_position(2)) && PX4_ISFINITE(_velocity(2));
 	return ret;
 }