sd: dispose SdStyleSheet properly

If it's disposed during document destruction it still retained pointers
to SfxItemSet and SfxStyleSheetBasePool.

Change-Id: I7b986b4fe5ca466f0fa86a99b744f5e2f85e08be
diff --git a/sd/source/core/stlsheet.cxx b/sd/source/core/stlsheet.cxx
index 4ed6289..050b1c0 100644
--- a/sd/source/core/stlsheet.cxx
+++ b/sd/source/core/stlsheet.cxx
@@ -773,6 +773,13 @@

void SdStyleSheet::disposing()
{
    SolarMutexGuard aGuard;
    if (bMySet)
    {
        delete pSet;
    }
    pSet = nullptr;
    pPool = nullptr;
    mxPool.clear();
}