Hack to work around an in-destruction SwTxtNode no longer being one

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

SwTxtFrm::GetTxtNode() const
SwFrm::InvalidatePage(SwPageFrm const*) const
SwCntntFrm::Cut()
SwCntntNode::DelFrms(bool)
SwCntntNode::~SwCntntNode()
SwTxtNode::~SwTxtNode()
SwNodes::RemoveNode(unsigned long, unsigned long, bool)
SwNodes::Delete(SwNodeIndex const&, unsigned long)
SwDoc::AppendDoc(SwDoc const&, unsigned short, SwPageDesc*, bool, int)
...

Change-Id: Ic3bdf067abba985a0b95d4f0a482a94098341198

sw/source/core/layout/wsfrm.cxx

@@ -435,13 +435,9 @@ void SwFrm::InvalidatePage( const SwPageFrm *pPage ) const
         }
         pRoot->SetIdleFlags();
 
-        const SwTxtFrm *pTxtFrm = dynamic_cast< const SwTxtFrm * >(this);
-        if (pTxtFrm)
-        {
-            const SwTxtNode *pTxtNode = pTxtFrm->GetTxtNode();
-            if (pTxtNode && pTxtNode->IsGrammarCheckDirty())
-                pRoot->SetNeedGrammarCheck( true );
-        }
+        const SwTxtNode *pTxtNode = dynamic_cast< const SwTxtNode * >(GetDep());
+        if (pTxtNode && pTxtNode->IsGrammarCheckDirty())
+            pRoot->SetNeedGrammarCheck( true );
     }
 }