make sure we have a bookmarks toolbar before we try to rebuild it

bug 342245
patch by Simon B��nzli
r=myk

browser/components/microsummaries/src/nsMicrosummaryService.js

@@ -683,6 +683,7 @@ MicrosummaryService.prototype = {
       
       // rebuild the bookmarks toolbar
       var bookmarksToolbar = win.document.getElementById("bookmarks-ptf");
+      if (bookmarksToolbar)
         bookmarksToolbar.builder.rebuild();
       
       // rebuild the bookmarks sidebar