Satisfy Coverity error

Change-Id: Ib619617adca0cb64fb69daffb3b57214e882d9b4
Reviewed-on: https://gerrit.libreoffice.org/53145Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

sw/source/core/crsr/trvltbl.cxx

@@ -590,6 +590,7 @@ bool GotoNextTable( SwPaM& rCurrentCursor, SwMoveFnCollection const & fnPosTable
             continue;
         }
 
+        if( pTableNd )  // should never be nullptr
         {
             if( &fnPosTable == &fnMoveForward ) // at the beginning?
             {