tdf#91523 sw: fix navigation to index mark

Access pTextTox instead of pTextField in case of TEXTTOXMARK

Change-Id: I92ece6f4c5d6a6427f89cbd43a15a4b0a3adb645
Reviewed-on: https://gerrit.libreoffice.org/18407Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>
Tested-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>

sw/source/core/doc/docfld.cxx

@@ -323,9 +323,11 @@ sal_Int32 _SetGetExpField::GetCntPosFromContent() const
         {
         case TEXTFIELD:
         case TEXTINET:
-        case TEXTTOXMARK:
             nRet = CNTNT.pTextField->GetStart();
             break;
+        case TEXTTOXMARK:
+            nRet = CNTNT.pTextTOX->GetStart();
+            break;
         case CRSRPOS:
             nRet =  CNTNT.pPos->nContent.GetIndex();
             break;