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

Merges [58470] to the 5.8 branch.
Props xknown, jorbin.



git-svn-id: https://develop.svn.wordpress.org/branches/5.8@58484 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -5788,6 +5788,9 @@ function iis7_supports_permalinks() {
  * @return int 0 means nothing is wrong, greater than 0 means something was wrong.
  */
 function validate_file( $file, $allowed_files = array() ) {
+	// Normalize path for Windows servers
+	$file = wp_normalize_path( $file );
+
 	// `../` on its own is not allowed:
 	if ( '../' === $file ) {
 		return 1;