Violation of principle that plan trees are read-only

While chasing down Valgrind leakage reports, I was disturbed
to realize that some of them arise from a case where the
executor scribbles on the plan tree it's given, which it is
absolutely not supposed to do:

/*
* Initialize result tuple slot and assign its rowtype using the first
* RETURNING list. We assume the rest will look the same.
*/
mtstate->ps.plan->targetlist = (List *) linitial(returningLists);

A bit of git archaeology fingers Andres' commit 4717fdb14, which we
can't easily revert since he later got rid of ExecAssignResultType
altogether. But I think we need to do something about it --- it's
purest luck that this doesn't cause serious problems in some cases.

regards, tom lane