sd: dispose SdStyleSheet properly

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

Change-Id: I7b986b4fe5ca466f0fa86a99b744f5e2f85e08be

sd/source/core/stlsheet.cxx

@@ -773,6 +773,13 @@ void SAL_CALL SdStyleSheet::dispose(  ) throw (RuntimeException, std::exception)
 
 void SdStyleSheet::disposing()
 {
+    SolarMutexGuard aGuard;
+    if (bMySet)
+    {
+        delete pSet;
+    }
+    pSet = nullptr;
+    pPool = nullptr;
     mxPool.clear();
 }