When creating a temporary file treat the `/` directory properly, to prevent it ending up in an endless self-calling loop.

Props hnle, taka2. Fixes #32135. See #31811
Merges [32322] to the 4.2 branch.

Built from https://develop.svn.wordpress.org/branches/4.2@32324


git-svn-id: http://core.svn.wordpress.org/branches/4.2@32295 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/file.php

@@ -146,7 +146,7 @@ function wp_tempnam( $filename = '', $dir = '' ) {
 		$dir = get_temp_dir();
 	}
 
-	if ( empty( $filename ) || '.' == $filename ) {
+	if ( empty( $filename ) || '.' == $filename || '/' == $filename ) {
 		$filename = time();
 	}