mmc: card: Avoid null pointer dereference

After the null check on md the code jumped to cmd_done, which then
will dereference md in mmc_blk_put. This patch avoids the possible
null pointer dereference in that case.

Signed-off-by: Philippe De Swert <philippedeswert@gmail.com>
Reviewed-by: Namjae Jeon <linkinjeon@gmail.com>
Signed-off-by: Chris Ball <cjb@laptop.org>

drivers/mmc/card/block.c

@@ -384,7 +384,7 @@ static int mmc_blk_ioctl_cmd(struct block_device *bdev,
 	md = mmc_blk_get(bdev->bd_disk);
 	if (!md) {
 		err = -EINVAL;
-		goto cmd_done;
+		goto cmd_err;
 	}
 
 	card = md->queue.card;
@@ -483,6 +483,7 @@ cmd_rel_host:
 
 cmd_done:
 	mmc_blk_put(md);
+cmd_err:
 	kfree(idata->buf);
 	kfree(idata);
 	return err;