CVE-2015-5299: s3-shadow-copy2: fix missing access check on snapdir

Fix originally from <partha@exablox.com>

https://bugzilla.samba.org/show_bug.cgi?id=11529

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: David Disseldorp <ddiss@samba.org>
