CVE-2018-16853: fix crash in expired passowrd case

CVE-2018-16853: fix crash in expired passowrd case

When calling encode_krb5_padata_sequence() make sure to
pass a null terminated array as required.

Fixes expired passowrd case in samba4.blackbox.kinit test.

Signed-off-by: Isaac Boukris <iboukris@gmail.com>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Autobuild-User(v4-8-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-8-test): Tue Dec  4 18:36:56 CET 2018 on sn-devel-144