Return false in get_attached_file() when the file does not exist, rather than...

Return false in get_attached_file() when the file does not exist, rather than a path to the base uploads directory. props mikeschinkel. fixes #18855.

git-svn-id: https://core.svn.wordpress.org/trunk@20613 1a063a9b-81f0-0310-95a4-ce76da25c4cd