Reviewed by Sam.

        <rdar://5585900>: Safari crashes when selected in context menu to open
        audio format files (au, aif) with QT 7.3

        The crash occurred on a machine where QT 7.3 was failing to initialize.
        The fix is to avoid sending streams to full-page plugins that've failed 
        to load

        * WebFrame.cpp:
        (WebFrame::finishedLoading): Check plugin status before calling manual
        stream methods
        (WebFrame::setMainDocumentError): Same
        (WebFrame::committedLoad): Same



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@27661 268f45cc-cd09-0410-ab3c-d52691b4dbfc