qemu-img: Fix leakage of options on error

Reported by Coverity.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-id: 20170515141014.25793-1-famz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
