skd: Make it easier for static analyzers to analyze skd_free_disk()

Although it is easy to see that skdev->disk != NULL if skdev->queue
!= NULL, add a test for skdev->disk to avoid that smatch reports the
following warning:

drivers/block/skd_main.c:3080 skd_free_disk()
         error: we previously assumed 'disk' could be null (see line 3074)
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/skd_main.c

@@ -3041,7 +3041,8 @@ static void skd_free_disk(struct skd_device *skdev)
 	if (skdev->queue) {
 		blk_cleanup_queue(skdev->queue);
 		skdev->queue = NULL;
-		disk->queue = NULL;
+		if (disk)
+			disk->queue = NULL;
 	}
 
 	if (skdev->tag_set.tags)