When visit_start_struct() succeeds, visit_end_struct() must be called.
hmp_object_add() doesn't when a member visit fails.  As far as I can
tell, the opts visitor copes okay with the misuse.  Fix it anyway.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>