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

Merges [58470] to the 5.6 branch.
Props xknown, jorbin.



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git-svn-id: http://core.svn.wordpress.org/branches/5.6@57935 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/functions.php

@@ -5436,6 +5436,9 @@ 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;