CVE-2015-5370: s3:rpc_server: don't allow an existing context to be changed in check_bind_req()

An alter context can't change the syntax of an existing context,
a new context_id will be used for that.

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>
