fixed a spurious 'notice' when used as library

bibtexbrowser.php

@@ -1379,7 +1379,7 @@ class BibEntry {
 
     // referrer, the id pf a collection of objects
     // see also http://www.openurl.info/registry/docs/pdf/info-sid.pdf    
-    $url_parts[]='rfr_id='.s3988('info:sid/'.$_SERVER['HTTP_HOST'].':'.@$_GET[Q_FILE]);
+    $url_parts[]='rfr_id='.s3988('info:sid/'.@$_SERVER['HTTP_HOST'].':'.@$_GET[Q_FILE]);
 
     $url_parts[]='rft.date='.s3988($this->getYear());