Reviewed by Adam.

        This fixes <rdar://problem/5060766> -- Frameset with 
        an empty frame crashes due to null document.  Debug
        builds hit the frame->document() assertion in 
        EventHandler::prepareMouseEvent

        This null check seems necessary now, having run through 
        all the logic on the path to this crash it appears that
        we can't avoid this null check.  

        * page/EventHandler.cpp:
        (WebCore::EventHandler::updateDragAndDrop):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@20868 268f45cc-cd09-0410-ab3c-d52691b4dbfc