s3:auth_builtin: anonymous authentication doesn't allow a password

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11847

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Günther Deschner <gd@samba.org>
