Reviewed by Ojan Vafai.

        REGRESSION(r83967): Crash in selectionExtentRespectingEditingBoundary
        https://bugs.webkit.org/show_bug.cgi?id=58910

        The crash was caused by selectionExtentRespectingEditingBoundary's incorrectly assuming that
        targetNode always have renderer when there selection has an editable root and the target node
        is outside of the editable root.
        
        Fixed the bug by adding an early exit when the target node is null.

        No new tests are added since we don't have a reduction for this crash.

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


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@84320 268f45cc-cd09-0410-ab3c-d52691b4dbfc