MDL-66966 tcpdf: Silence non-existing K_PATH_CACHE

I was fixing this adding a extra dir_exists() check that
is better than the silence operator but that was that already
there was a PR using @. So went to it.

This is https://git.io/JeKft

lib/tcpdf/tcpdf.php

@@ -7788,7 +7788,7 @@ public function _destroy($destroyall=false, $preserve_objcopy=false) {
 		if ($destroyall AND !$preserve_objcopy) {
 			self::$cleaned_ids[$this->file_id] = true;
 			// remove all temporary files
-			if ($handle = opendir(K_PATH_CACHE)) {
+			if ($handle = @opendir(K_PATH_CACHE)) {
 				while ( false !== ( $file_name = readdir( $handle ) ) ) {
 					if (strpos($file_name, '__tcpdf_'.$this->file_id.'_') === 0) {
 						unlink(K_PATH_CACHE.$file_name);