CVE-2016-2112: s3:libads: make sure we detect downgrade attacks

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11644

Pair-programmed-with: Ralph Boehme <slow@samba.org>

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Signed-off-by: Ralph Boehme <slow@samba.org>
