fdo#76361 : LO crash: LO crashes while saving the document to docx

The document contains an empty table with borders.
LO was treating this table as a nested table, which lead to
a TC tag mismatch.
In docxattributeoutput.cxx, inside 'switchHeaderFooter'
added a check against 'm_oldTableReference->m_pOldTablepInner'
For some documents, even if the above value was null, still
it was getting assigned to 'm_tableReference' . Thus in such situations
it leads to a crash.

Change-Id: I883267585cb804f961989797de57c4b843b96821
Reviewed-on: https://gerrit.libreoffice.org/8677Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>

sw/qa/extras/ooxmlexport/ooxmlexport.cxx

@@ -2887,6 +2887,18 @@ DECLARE_OOXMLEXPORT_TEST(test76108, "test76108.docx")
     assertXPath(pXmlDoc, "/w:document[1]/w:body[1]/w:p[1]/w:r[1]/w:fldChar[1]", "fldCharType", "begin");
 }
 
+DECLARE_OOXMLEXPORT_TEST(testTCTagMisMatch, "TCTagMisMatch.docx")
+{
+   // TCTagMisMatch.docx : This document contains a empty table with borders.
+   // there was a TC tag mismatch which resulted into a crash.
+
+   xmlDocPtr pXmlDoc = parseExport("word/document.xml");
+   if(!pXmlDoc)
+      return;
+   assertXPath(pXmlDoc,"/w:document[1]/w:body[1]/w:tbl[1]/w:tr[1]/w:tc[1]/w:tbl[1]/w:tr[1]/w:tc[1]",0);
+   assertXPath(pXmlDoc,"/w:document[1]/w:body[1]/w:tbl[1]/w:tr[1]/w:tc[1]", 1);
+}
+
 DECLARE_OOXMLEXPORT_TEST(testHyperLinkTagEnded, "fdo76316.docx")
 {
     /* XML tag <w:hyperlink> was not getting closed when its inside another

sw/source/filter/ww8/docxattributeoutput.cxx

@@ -2489,10 +2489,15 @@ void DocxAttributeOutput::switchHeaderFooter(bool isHeaderFooter, sal_Int32 inde
     }
     else if( index == -1)
     {
-        *m_tableReference = *m_oldTableReference;
-        //Reset the oldReference, after copying it back to the original.
-        m_oldTableReference->m_bTableCellOpen = false ;
-        m_oldTableReference->m_nTableDepth = 0;
+       if (m_oldTableReference->m_pOldTablepInner)
+       {
+           *m_tableReference = *m_oldTableReference;
+
+            //Reset the oldReference, after copying it back to the original.
+            m_oldTableReference->m_bTableCellOpen = false ;
+            m_oldTableReference->m_nTableDepth = 0;
+       }
+
     }
     else
     {