fmu: fix safety button

- use the orb topic safety to check the safety state, because the actual
  safety state can come from the IO
- fix initialization if circuit breaker is set

src/drivers/px4io/px4io.cpp

@@ -1650,7 +1650,8 @@ PX4IO::io_handle_status(uint16_t status)
 		orb_publish(ORB_ID(safety), _to_safety, &safety);
 
 	} else {
-		_to_safety = orb_advertise(ORB_ID(safety), &safety);
+		int instance;
+		_to_safety = orb_advertise_multi(ORB_ID(safety), &safety, &instance, ORB_PRIO_DEFAULT);
 	}
 
 	return ret;