qapi: Add missing null check to opts_start_struct()

Argument is null when visiting an unboxed struct.  I can't see such a
visit in the current code.  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>
