Fix <rdar://problem/5092653>
        REGRESSION: Image decoding making scrolling slow
        
        If decoder gave back a null frame (which I think happens regularly if you don't have full frame 
        data yet for image that does not decode incrementally) m_decodedSize was incremented anyway. This
        led to massive growth in calculated image size and eventually made cache prune constantly. 
        
        No test, I don't know how to make one for this.

        * platform/graphics/BitmapImage.cpp:
        (WebCore::BitmapImage::cacheFrame):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@20558 268f45cc-cd09-0410-ab3c-d52691b4dbfc