more sanity for getPath

$path has to be a file, not a directory (previously e.g. when being
passed an empty string, it returned WP_ROOT_DIR), kudo's to @zytzagoo
:-)

classes/autoptimizeBase.php

@@ -104,7 +104,7 @@ protected function getpath($url) {
         $path = str_replace('//','/',WP_ROOT_DIR.$path);
 
         // final check: does file exist and is it readable
-        if (file_exists($path) && is_readable($path)) {
+        if (file_exists($path) && is_file($path) && is_readable($path)) {
             return $path;
         } else {
             return false;