CVE-2015-5370: s3:rpc_server: make use of pipe_auth_data->auth_context_id

This is better than using hardcoded values.
We need to use the value the client used in the BIND request.

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>
