CVE-2015-5370: s4:rpc_server: no authentication is indicated by pkt->auth_length == 0

pkt->u.*.auth_info.length is not the correct thing to check.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11344

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