Assert failure in find_var_for_subquery_tle

The following query, run in the regression database, trips an Assert:

select ss.v from int8_tbl x,
lateral (select x.q1 from (select distinct q2 from int8_tbl) s
union all
select q2 from int8_tbl) ss(v);

TRAP: failed Assert("var->varno == rel->relid"), File: "pathkeys.c"

In set_append_rel_size(), we have this comment:

* NB: the resulting childrel->reltarget->exprs may contain arbitrary
* expressions, which otherwise would not occur in a rel's targetlist.
* Code that might be looking at an appendrel child must cope with
* such. (Normally, a rel's targetlist would only include Vars and
* PlaceHolderVars.)

Apparently find_var_for_subquery_tle() did not get this memo, and
assumes that every Var in a subquery rel's targetlist belongs to that
rel.

To fix, I think we can just ignore Vars that do not belong to the
subquery rel. Such a Var cannot be a reference to the subquery's
output, so it should never be treated as a match.

Any thoughts?

- Richard