CVE-2015-0240: s3: netlogon: Ensure we don't call talloc_free on an uninitialized pointer.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=11077

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
