Editor: Fix Path Traversal issue on Windows in Template-Part Block.

Merges [58470] to the 4.8 branch.
Props xknown, jorbin.




git-svn-id: https://develop.svn.wordpress.org/branches/4.8@58494 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -4354,6 +4354,9 @@ function iis7_supports_permalinks() {
  * @return int 0 means nothing is wrong, greater than 0 means something was wrong.
  */
 function validate_file( $file, $allowed_files = '' ) {
+	// Normalize path for Windows servers
+	$file = wp_normalize_path( $file );
+
 	if ( false !== strpos( $file, '..' ) )
 		return 1;