Fix for <rdar://problem/4621660> REGRESSION: Safari crashing in 
        WebCore::FrameView::updateOverflowStatus

        m_viewportRenderer is never initialized for framesets, and it 
        shouldn't be. So we just need to nil-check for it in 
        updateOverflowStatus() and return early.

        * page/FrameView.cpp:
        (WebCore::FrameView::updateOverflowStatus): Nil check.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@15433 268f45cc-cd09-0410-ab3c-d52691b4dbfc