ST_EstimatedExtent gives bogus results on combined gist indexes

[Björn Harrtell]

Reproduction:

drop table if exists test;
with vals(k, geom) as (values (1, 'POINT(1 1)'::geometry), (2, 'POINT(1 2)'::geometry))
select * into table test from vals;
analyze test;
select st_estimatedextent('test', 'geom');
-- BOX(1 0.995000004768372,1 2.005000114440918)
create index test_geom_idx on test using gist (geom);
select st_estimatedextent('test', 'geom');
-- BOX(1 1,1 2)
drop index test_geom_idx;
create index test_k_geom_idx on test using gist (k, geom);
select st_estimatedextent('test', 'geom');
-- BOX(1.401298464324817E-45 1,2.802596928649634E-45 1)

This is on:

POSTGIS="3.2.0dev 3.2.0beta2-11-g27f44ecf6" [EXTENSION] PGSQL="120" GEOS="3.9.0-CAPI-1.16.2" PROJ="7.2.1" LIBXML="2.9.12" LIBJSON="0.15" (core procs from "3.2.0dev 3.2.0beta2-3-geda70258f" need upgrade)

But I think it's an old bug.