block: xen-blkfront: Fix possible NULL ptr dereference

In the blkif_release function the bdget_disk() call might returns
a NULL ptr which might be dereferenced on bdev->bd_openers checking
Signed-off-by: Felipe Pena <felipensp@gmail.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
[v2: Added WARN per Roger's suggestion]

drivers/block/xen-blkfront.c

@@ -2011,6 +2011,10 @@ static void blkif_release(struct gendisk *disk, fmode_t mode)
 
 	bdev = bdget_disk(disk, 0);
 
+	if (!bdev) {
+		WARN(1, "Block device %s yanked out from us!\n", disk->disk_name);
+		goto out_mutex;
+	}
 	if (bdev->bd_openers)
 		goto out;
 
@@ -2041,6 +2045,7 @@ static void blkif_release(struct gendisk *disk, fmode_t mode)
 
 out:
 	bdput(bdev);
+out_mutex:
 	mutex_unlock(&blkfront_mutex);
 }