CVE-2015-5370: s3:librpc/rpc: make use of auth->auth_context_id in dcerpc_add_auth_footer()

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>
