check for TableBox without StartNode

Change-Id: I77b21c85716083029ab8873094621e0f8eceb48f

sw/source/filter/ww8/docxattributeoutput.cxx

@@ -3597,7 +3597,10 @@ void DocxAttributeOutput::TableVerticalCell( ww8::WW8TableNodeInfoInner::Pointer
     else if ( FRMDIR_HORI_LEFT_TOP == m_rExport.TrueFrameDirection( *pFrmFmt ) )
     {
         // Undo the text direction mangling done by the btLr handler in writerfilter::dmapper::DomainMapperTableManager::sprm()
-        SwPaM aPam(*pTabBox->GetSttNd(), 0);
+        const SwStartNode* pSttNd = pTabBox->GetSttNd();
+        if (pSttNd)
+        {
+            SwPaM aPam(*pSttNd, 0);
             ++aPam.GetPoint()->nNode;
             if (aPam.GetPoint()->nNode.GetNode().IsTxtNode())
             {
@@ -3613,6 +3616,7 @@ void DocxAttributeOutput::TableVerticalCell( ww8::WW8TableNodeInfoInner::Pointer
                 }
             }
         }
+    }
 
     const SwWriteTableRows& aRows = m_xTableWrt->GetRows( );
     SwWriteTableRow *pRow = aRows[ pTableTextNodeInfoInner->getRow( ) ];