Hack to make an in-destruction SwFlyFrm no longer claim to be one

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

SwFrm::FindPageFrm()
SwAnchoredObject::FindPageFrmOfAnchor()
lcl_NotifyBackgroundOfObj(SwDrawContact&, SdrObject const&, Rectangle const*)
SwDrawContact::DisconnectFromLayout(bool)
SwDrawContact::DisconnectObjFromLayout(SdrObject*)
SwLayoutFrm::Destroy()
SwLayoutFrm::~SwLayoutFrm()
SwFlyFrm::~SwFlyFrm()
SwFlyFreeFrm::~SwFlyFreeFrm()
SwFlyLayFrm::~SwFlyLayFrm()
SwLayoutFrm::Destroy()
SwRootFrm::~SwRootFrm()
...
boost::shared_ptr<SwRootFrm>::~shared_ptr()
SwViewShell::~SwViewShell()
SwCrsrShell::~SwCrsrShell()
SwEditShell::~SwEditShell()
SwFEShell::~SwFEShell()
SwWrtShell::~SwWrtShell()
SwView::~SwView()
SfxViewFrame::ReleaseObjectShell_Impl()
SfxViewFrame::~SfxViewFrame()
SfxViewFrame::Close()
...

Change-Id: I56d7135044e8682eeaf6ef23c76d4d7beca1eff4

sw/source/core/layout/fly.cxx

@@ -268,6 +268,10 @@ SwFlyFrm::~SwFlyFrm()
     }
 
     FinitDrawObj();
+
+    // Hack to make sure code called from base ~SwLayoutFrm does not interpret
+    // this as a SwFlyFrm (which it no longer is by then):
+    mnFrmType = FRM_UNUSED;
 }
 
 const IDocumentDrawModelAccess* SwFlyFrm::getIDocumentDrawModelAccess()