rt2x00: fix a possible NULL pointer dereference

The 'rt2x00lib_probe_dev' function tries to
allocate the workqueue. If the allocation
fails, 'rt2x00_lib_remove_dev' is called on
the error path. Because 'rt2x00dev->workqueue'
is NULL in this case, the 'destroy_workqueue'
call will cause a NULL pointer dereference.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rt2x00/rt2x00dev.c

@@ -1220,7 +1220,8 @@ void rt2x00lib_remove_dev(struct rt2x00_dev *rt2x00dev)
 		cancel_work_sync(&rt2x00dev->rxdone_work);
 		cancel_work_sync(&rt2x00dev->txdone_work);
 	}
-	destroy_workqueue(rt2x00dev->workqueue);
+	if (rt2x00dev->workqueue)
+		destroy_workqueue(rt2x00dev->workqueue);
 
 	/*
 	 * Free the tx status fifo.