Restore code that returns the existing media object if it already exists.

props programmin, adamsilverstein. fixes #24062

git-svn-id: http://core.svn.wordpress.org/trunk@24447 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Fix storage of illegal_names. Add an upgrade routine to fix bad values.

props SergeyBiryukov.
see #23418.
for trunk.



git-svn-id: http://core.svn.wordpress.org/trunk@24448 1a063a9b-81f0-0310-95a4-ce76da25c4cd