CVE-2015-5370: s4:librpc/rpc: handle DCERPC_PKT_FAULT before anything else in dcerpc_alter_context_recv_handler()

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>
