CVE-2015-5370: s3:rpc_server: verify auth_context_id in api_pipe_{bind_auth3,alter_context}

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>
