New test doc-bad-union-member.json shows we can fail to reject
documentation for nonexistent members.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1489582656-31133-37-git-send-email-armbru@redhat.com>