fix SEGV in CppunitTest_sw_core_undo on Windows

when something else changed the clipboard concurrently
 CppunitTest_sw_core_undo

Change-Id: Ie0d21ed77ac38bfe2ca55e5b001e3fd09ac05493
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/107159
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
diff --git a/sw/source/uibase/dochdl/swdtflvr.cxx b/sw/source/uibase/dochdl/swdtflvr.cxx
index 9200cd6..66ba7e6 100644
--- a/sw/source/uibase/dochdl/swdtflvr.cxx
+++ b/sw/source/uibase/dochdl/swdtflvr.cxx
@@ -315,6 +315,8 @@ static SwDoc& lcl_GetDoc(SwDocFac & rDocFac)
void SwTransferable::ObjectReleased()
{
    SwModule *pMod = SW_MOD();
    if (!pMod)
        return;
    if( this == pMod->m_pDragDrop )
        pMod->m_pDragDrop = nullptr;
    else if( this == pMod->m_pXSelection )