coverity#1433644 silence Dereference after null check

Change-Id: I699fb3e07d21dc1852c0d50d4a7d478bcf1d1a96
Reviewed-on: https://gerrit.libreoffice.org/52063Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

sd/source/ui/slidesorter/controller/SlsClipboard.cxx

@@ -460,6 +460,8 @@ void Clipboard::CreateSlideTransferable (
                 pActionWindow = pViewShell->GetActiveWindow();
         }
 
+        assert(pActionWindow);
+
         pTransferable->SetStartPos (pActionWindow->PixelToLogic(
             pActionWindow->GetPointerPosPixel()));
         pTransferable->SetObjectDescriptor (std::move(pObjDesc));