ASAN/valgrind errors in connect.misc test

accessing freed memory.
Before XMLCOL::WriteColumn() Tdbp->Clist gets assigned
a nodelist in

      Clist = RowNode->SelectNodes(g, Colname, Clist);

which is RowNode->Doc->Xop->nodesetval.

In XMLCOL::WriteColumn()

        ValNode = ColNode->SelectSingleNode(g, Xname, Vxnp);

calls LIBXMLDOC::GetNodeList() again, which frees the previous
XPath object Xop and replaces it with a new one.

In this case RowNode->Doc == ColNode->Doc, so Clist->Listp
points to a freed memory now.

storage/connect/tabxml.cpp

@@ -1803,6 +1803,9 @@ void XMLCOL::WriteColumn(PGLOBAL g)
     else if (Tdbp->Clist)
       ColNode = NULL;
 
+    // refresh CList in case its Listp was freed in SelectSingleNode above
+    if (Tdbp->Clist)
+      Tdbp->RowNode->SelectNodes(g, Tdbp->Colname, Tdbp->Clist);
     } // endfor i
 
   /*********************************************************************/