[PATCH] nodeindexscan with reorder memory leak

Hey!

I was investigating a leak reported in the PostGIS issues tracker [1] which
led me to the Postgres side where the problem really is. The leak is
reproducible with query from original ticket [1]:

WITH latitudes AS (
SELECT generate_series AS latitude
FROM generate_series(-90, 90, 0.1)
), longitudes AS (
SELECT generate_series AS longitude
FROM generate_series(-180, 180, 0.1)
), points AS (
SELECT ST_SetSRID(ST_Point(longitude, latitude), 4326)::geography AS geog
FROM latitudes
CROSS JOIN longitudes
)
SELECT
geog,
(
SELECT name
FROM ne_110m_admin_0_countries AS ne
ORDER BY p.geog <-> ne.geog
LIMIT 1
)
FROM points AS p
;

The leak is only noticeable when index scan with reorder happens as part of
subquery plan which is explained by the fact that heap tuples cloned in
reorderqueue_push are not freed during flush of reorder queue in
ExecReScanIndex.

[1] https://trac.osgeo.org/postgis/ticket/4720