fdo#71866: sw: un-break index updates

SwTOXBaseSection::GenerateText() wants to operate directly on
SwTxtNode::m_Text and accepts no substitutes such as a temporary string
on the stack; this regression shows that developers often
significantly underestimate the craziness of the existing code.

(regression from d6e321f8)

Change-Id: I01b831f35a52bde207a7db8cae9d4aadb826b2d8

sw/source/core/doc/doctxm.cxx

@@ -1583,7 +1583,8 @@ void SwTOXBaseSection::GenerateText( sal_uInt16 nArrayIdx,
 
     // pTOXNd is only set at the first mark
     SwTxtNode* pTOXNd = (SwTxtNode*)aSortArr[nArrayIdx]->pTOXNd;
-    OUString rTxt = pTOXNd->GetTxt();
+    // FIXME this operates directly on the node text
+    OUString & rTxt = const_cast<OUString&>(pTOXNd->GetTxt());
     rTxt = "";
     for(sal_uInt16 nIndex = nArrayIdx; nIndex < nArrayIdx + nCount; nIndex++)
     {