Fix freeing of dangling IndexScanDesc.xs_hitup in GiST

Hello, hackers!

The last query in the following script crashes Postgres:

create table t (id serial, amount int);
insert into t (amount) select random() * 1000 from generate_series(1, 100);
create extension btree_gist;
create index t_gist_idx on t using gist(id, amount);

select p.id, p.amount, s.nearest
from t as p left join lateral
(
select p.id, array_agg(l.id) as nearest from (
select id from t order by amount <-> p.amount limit 10
) l
) s using(id);

In gistrescan() IndexScanDesc.xs_hitup is not reset after MemoryContextReset() of
so->queueCxt in which xs_hitup was allocated, then getNextNearest() tries to pfree()
dangling xs_hitup, which results in the reuse of this pointer and the subsequent crash.

Attached patches fix this bug introduced in commit
d04c8ed9044eccebce043143a930617e3998c005 "Add support for index-only scans in GiST".
The bug is present in v9.5, v9.6, v10.0.

--
Nikita Glukhov
Postgres Professional:http://www.postgrespro.com
The Russian Postgres Company