[PATCH] USB: add better check to prevent oops in hcd_unlink_urb()

drivers/usb/core/hcd.c

@@ -1229,7 +1229,7 @@ static int hcd_unlink_urb (struct urb *urb)
 	spin_unlock (&hcd_data_lock);
 	spin_unlock_irqrestore (&urb->lock, flags);
 bye:
-	if (retval && sys)
+	if (retval && sys && sys->driver)
 		dev_dbg (sys, "hcd_unlink_urb %p fail %d\n", urb, retval);
 	return retval;
 }