qcow2: Fix memory leak in COW error path

This triggers if bs->drv becomes NULL in a concurrent request. This is
currently only the case when corruption prevention kicks in (i.e. at
most once per image, and after that it produces I/O errors).

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block/qcow2-cluster.c

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 76d2bcf..4208dc0 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -379,7 +379,8 @@
     BLKDBG_EVENT(bs->file, BLKDBG_COW_READ);
 
     if (!bs->drv) {
-        return -ENOMEDIUM;
+        ret = -ENOMEDIUM;
+        goto out;
     }
 
     /* Call .bdrv_co_readv() directly instead of using the public block-layer