as observed by -fsanitize=vptr e.g. during CppunitTest_sw_mailmerge (though the
true fix might be to prevent all this from happening during ~SwPageFrm anyway?):

SwFrm::FindPageFrm()
SwAnchoredObject::FindPageFrmOfAnchor()
lcl_NotifyBackgroundOfObj(SwDrawContact&, SdrObject const&, Rectangle const*)
SwDrawContact::DisconnectFromLayout(bool)
SwDrawContact::DisconnectObjFromLayout(SdrObject*)
SwLayoutFrm::Destroy()
SwLayoutFrm::~SwLayoutFrm()
SwFtnBossFrm::~SwFtnBossFrm()
SwPageFrm::~SwPageFrm()
SwFrm::CheckPageDescs(SwPageFrm*, bool, SwPageFrm**)
SwFlowFrm::MoveBwd(bool&)
SwCntntFrm::MakeAll()
SwFrm::PrepareMake()
SwFrm::Calc()
SwLayAction::IsShortCut(SwPageFrm*&)
SwLayAction::InternalAction()
SwLayAction::Action()
SwViewShell::ImplEndAction(bool)
SwViewShell::EndAction(bool)
SwCrsrShell::EndAction(bool, bool)
SwEditShell::EndAllAction()
SwDoc::AppendDoc(SwDoc const&, unsigned short, SwPageDesc*, bool)
...

Change-Id: Iac207ef4a80318609f0a00d219dcccacaee7b205