operations: Fix crash when no usb device is detected

src/operations.c

@@ -80,7 +80,8 @@ struct device_info * dev_detect(void) {
 	}
 
 clean:
-	usb_close_device(usb);
+	if (usb)
+		usb_close_device(usb);
 	free(dev);
 	return NULL;