CVE-2015-5370: s4:rpc_server: fix the order of error checking in dcesrv_alter()

The basically matches Windows 2012R2, it's not 100%
but it's enough for our raw protocol tests to pass.

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>
