Fix error paths if md_probe fails.

md_probe can fail (e.g. alloc_disk could fail) without
returning an error (as it alway returns NULL).
So when we call mddev_find immediately afterwards, we need
to check that md_probe actually succeeded.  This means checking
that mdev->gendisk is non-NULL.

cc: <stable@kernel.org>
Cc: Dave Jones <davej@redhat.com>
Signed-off-by: Neil Brown <neilb@suse.de>

drivers/md/md.c

@@ -3897,7 +3897,9 @@ static void autorun_devices(int part)
 
 		md_probe(dev, NULL, NULL);
 		mddev = mddev_find(dev);
-		if (!mddev) {
+		if (!mddev || !mddev->gendisk) {
+			if (mddev)
+				mddev_put(mddev);
 			printk(KERN_ERR
 				"md: cannot allocate memory for md drive.\n");
 			break;