MAGETWO-44919: [HHVM] hphp_invoke error when Web Setup

- ignoring validateDOM, if libxml_set_external_entity_loader does not exist, for eg for HHVM

Author: mazhalai

lib/internal/Magento/Framework/Config/Dom.php

@@ -276,6 +276,10 @@ public static function validateDomDocument(
         $schema,
         $errorFormat = self::ERROR_FORMAT_DEFAULT
     ) {
+        if (!function_exists('libxml_set_external_entity_loader')) {
+            return [];
+        }
+
         if (!self::$urnResolver) {
             self::$urnResolver = new UrnResolver();
         }