Avoid warning/notice spew in XRD parser. Not all DOM nodes are elements.

plugins/OStatus/lib/xrd.php

@@ -149,11 +149,13 @@ class XRD
         $link['href'] = $element->getAttribute('href');
         $link['template'] = $element->getAttribute('template');
         foreach ($element->childNodes as $node) {
+            if ($node instanceof DOMElement) {
                 switch($node->tagName) {
                 case 'Title':
                     $link['title'][] = $node->nodeValue;
                 }
             }
+        }
 
         return $link;
     }