Only check the post parent if it exists

See #272

git-svn-id: http://gsoc.svn.wordpress.org/2013/rmccue/trunk@2289 f69abe3c-2ee2-4550-9a6d-17bd458115be
Unattached attachments are assumed to be public

See #272

git-svn-id: http://gsoc.svn.wordpress.org/2013/rmccue/trunk@2290 f69abe3c-2ee2-4550-9a6d-17bd458115be