add file_exists check for cache delete

need to eliminate php warnings

Author: mipapo

phpfastcache/3.0.0/drivers/files.php

@@ -118,7 +118,7 @@ function driver_get($keyword, $option = array()) {
 
     function driver_delete($keyword, $option = array()) {
         $file_path = $this->getFilePath($keyword,true);
-        if(@unlink($file_path)) {
+        if(file_exists($file_path) && @unlink($file_path)) {
             return true;
         } else {
             return false;