vcl: PVS-Studio V595 'mpImplWin' pointer could be null

... but actually it's not if mpFloatWin isn't.

Change-Id: If26444e94221efb9921e94496905834b3ace7d28

vcl/source/control/lstbox.cxx

@@ -1465,6 +1465,7 @@ void ListBox::SetEdgeBlending(bool bNew)
 
         if(IsDropDownBox())
         {
+            assert(mpImplWin);
             mpImplWin->Invalidate();
         }
         else