CVE-2015-5370: s4:rpc_server: ensure that the message ordering doesn't violate the spec

The first pdu is always a BIND.

REQUEST pdus are only allowed once the authentication
is finished.

A simple anonymous authentication is finished after the BIND.
Real authentication may need additional ALTER or AUTH3 exchanges.

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>
