Prevent downcast to SwTxtFrm when already in ~SwFrm superclass dtor

...as done in SwAnchoredObject::FindAnchorCharFrm
(sw/source/core/layout/anchoredobject.cxx) during CppunitTest_sw_ooxmlexport5:

SwAnchoredObject::FindAnchorCharFrm
SwAnchoredObject::GetAnchorFrmContainingAnchPos
SwAnchoredObject::FindPageFrmOfAnchor
lcl_NotifyBackgroundOfObj
SwDrawContact::DisconnectFromLayout
SwDrawContact::DisconnectObjFromLayout
SwFrm::Destroy
SwFrm::~SwFrm
SwCntntFrm::~SwCntntFrm
SwTxtFrm::~SwTxtFrm
SwTxtFrm::~SwTxtFrm
SwLayoutFrm::Destroy
SwLayoutFrm::~SwLayoutFrm
SwCellFrm::~SwCellFrm
SwCellFrm::~SwCellFrm
SwLayoutFrm::Destroy
SwLayoutFrm::~SwLayoutFrm
SwRowFrm::~SwRowFrm
SwRowFrm::~SwRowFrm
SwLayoutFrm::Destroy
SwLayoutFrm::~SwLayoutFrm
SwTabFrm::~SwTabFrm
SwTabFrm::~SwTabFrm
SwFlyFrm::DeleteCnt
SwFlyFrm::~SwFlyFrm
SwFlyFreeFrm::~SwFlyFreeFrm
SwFlyAtCntFrm::~SwFlyAtCntFrm
SwFlyAtCntFrm::~SwFlyAtCntFrm
SwLayoutFrm::Destroy
SwLayoutFrm::~SwLayoutFrm
SwBodyFrm::~SwBodyFrm
SwBodyFrm::~SwBodyFrm
SwLayoutFrm::Destroy
SwLayoutFrm::~SwLayoutFrm
SwFtnBossFrm::~SwFtnBossFrm
SwPageFrm::~SwPageFrm
SwPageFrm::~SwPageFrm
SwLayoutFrm::Destroy
SwRootFrm::~SwRootFrm
SwRootFrm::~SwRootFrm
boost::checked_delete<SwRootFrm>
boost::detail::sp_counted_impl_p<SwRootFrm>::dispose
boost::detail::sp_counted_base::release
boost::detail::shared_count::~shared_count
boost::shared_ptr<SwRootFrm>::~shared_ptr
SwViewShell::~SwViewShell
SwCrsrShell::~SwCrsrShell
SwEditShell::~SwEditShell
SwFEShell::~SwFEShell
SwWrtShell::~SwWrtShell
SwWrtShell::~SwWrtShell
SwView::~SwView
SwView::~SwView
SfxViewFrame::ReleaseObjectShell_Impl
SfxViewFrame::~SfxViewFrame
SfxViewFrame::~SfxViewFrame
SfxViewFrame::Close
SfxFrame::DoClose_Impl
SfxBaseController::dispose
(anonymous namespace)::Frame::setComponent
(anonymous namespace)::Frame::close
SfxFrame::DoClose
SfxViewFrame::Notify
SfxBroadcaster::Broadcast
SfxModelListener_Impl::notifyClosing
SfxBaseModel::close
SwXTextDocument::close
SfxBaseModel::dispose
SwXTextDocument::dispose
SwModelTestBase::tearDown
...

Change-Id: I4c00fa8eebf0346a2bad81a9e362cd901555bbf0