tdf#137008 mml crash on load

Change-Id: I1d35f1b6cb7234850c6847b19848be186f2e1955
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/103370
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

starmath/source/visitors.cxx

@@ -2335,7 +2335,8 @@ void SmNodeToTextVisitor::Visit( SmMatrixNode* pNode )
         {
             SmNode* pSubNode = pNode->GetSubNode( i * pNode->GetNumCols( ) + j );
             Separate( );
-            pSubNode->Accept( this );
+            if (pSubNode)
+                pSubNode->Accept( this );
             Separate( );
             if (j != pNode->GetNumCols() - 1U)
                 Append( "#" );