snapshot: Reset err to NULL to avoid double free

If an error occurs in bdrv_snapshot_delete_by_id_or_name(), "err" is
freed.  If "err" is not set to NULL before calling
bdrv_snapshot_delete_by_id_or_name() again, it will not be updated on
error, and will be freed again.

This can be triggered by starting a VM with at least two drives and then
attempting to delete a non-existent snapshot.

Broken in commit a89d89d.

Signed-off-by: Chris Spiegel <chris.spiegel@cypherpath.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-id: 1412613225-32676-1-git-send-email-chris.spiegel@cypherpath.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
