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>
