Avoid PHP notices in Media Library if an attachment parent does not exist. props trepmal, viniciusmassuchetto, alex-ye, adamsilverstein for initial patches. fixes #22312. fixes #23154.

git-svn-id: http://core.svn.wordpress.org/trunk@24182 1a063a9b-81f0-0310-95a4-ce76da25c4cd