protect against double dispose.

Change-Id: Ib168188f7cfd8d4e0e6fe0617c5c9b7de840016b
diff --git a/svtools/source/contnr/ivctrl.cxx b/svtools/source/contnr/ivctrl.cxx
index 1baa80c..3e8db3b 100644
--- a/svtools/source/contnr/ivctrl.cxx
+++ b/svtools/source/contnr/ivctrl.cxx
@@ -93,8 +93,12 @@ SvtIconChoiceCtrl::~SvtIconChoiceCtrl()

void SvtIconChoiceCtrl::dispose()
{
    _pImp->CallEventListeners( VCLEVENT_OBJECT_DYING );
    delete _pImp;
    if (_pImp)
    {
        _pImp->CallEventListeners( VCLEVENT_OBJECT_DYING );
        delete _pImp;
        _pImp = NULL;
    }
    Control::dispose();
}