CVE-2015-5370: s4:rpc_server: verify the protocol headers before processing pdus

On protocol errors we should send BIND_NAK or FAULT and mark the
connection as to be terminated.

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>
