Quick-and-dirty fix for recursive plpgsql functions, per bug report from

Frank Miles 7-Sep-01.  This is really just sticking a finger in the dike.
Frank's case works now, but we still couldn't support a recursive function
returning a set.  Really need to restructure querytrees and execution
state so that the querytree is *read only*.  We've run into this over and
over and over again ... it has to happen sometime soon.