[sqlsmith] ERROR: plan should not reference subplan's variable

Updating master from f8c5855..1bdae16, sqlsmith triggers "failed to
generate plan" errors again. Below is the smallest query logged so far.

regards,
Andreas

-- ERROR: plan should not reference subplan's variable
set force_parallel_mode = 'on';
set max_parallel_workers_per_gather = '1';

explain WITH
jennifer_0 AS (select
(select b from public.rtest_v1 limit 1 offset 5)
as c0,
pg_catalog.pg_current_xlog_location() as c1,
sample_0.a as c2,
sample_0.a as c3
from
public.rtest_view4 as sample_0 tablesample system (5.9)
where cast(null as bigint) = pg_catalog.hashinet(
cast((select client_addr from pg_catalog.pg_stat_activity limit 1 offset 35)
as inet))
limit 76)
select
ref_0.sl_name as c0
from
public.shoelace as ref_0
where (cast(null as anyrange) < cast(null as anyrange))
and (EXISTS (
select
39 as c0
from
jennifer_0 as ref_1
where cast(null as real) = cast(null as real)
limit 81))
limit 96;