sd: AccessibleSlideSorterView: don't register twice at ViewShell

Triggers assert in SfxListener::StartListening()

Change-Id: I17f165d600fa789d7ce8d7b2296988064c5d5844
diff --git a/sd/source/ui/accessibility/AccessibleSlideSorterView.cxx b/sd/source/ui/accessibility/AccessibleSlideSorterView.cxx
index b530e3e..8268c55 100644
--- a/sd/source/ui/accessibility/AccessibleSlideSorterView.cxx
+++ b/sd/source/ui/accessibility/AccessibleSlideSorterView.cxx
@@ -796,8 +796,11 @@ void AccessibleSlideSorterView::Implementation::ReleaseListeners()

    if (mbListeningToDocument)
    {
        if (mrSlideSorter.GetViewShell() != nullptr)
        if (mrSlideSorter.GetViewShell() != nullptr && !IsListening(*mrSlideSorter.GetViewShell()))
        {   // ??? is it even possible that ConnectListeners is called with no
            // view shell and this one with a view shell?
            StartListening(*mrSlideSorter.GetViewShell());
        }
        EndListening (*mrSlideSorter.GetModel().GetDocument());
        mbListeningToDocument = false;
    }