WP_Filesyste: SSH2 PHP Extension support: Fix a case where defining a custom FS_CHMOD_FILE / FS_CHMOD_DIR constant wouldn't take effect due to chmod() failing with a invalid path. Fixes #23589

Built from https://develop.svn.wordpress.org/trunk@25306


git-svn-id: http://core.svn.wordpress.org/trunk@25268 1a063a9b-81f0-0310-95a4-ce76da25c4cd