BUG #19780: gtsvector_picksplit() reads uninitialized data when called with three entries

The following bug has been logged on the website:

Bug reference: 19780
Logged by: Alexander Lakhin
Email address: exclusion(at)gmail(dot)com
PostgreSQL version: 19rc1
Operating system: Ubuntu 24.04
Description:

The following script:
CREATE TABLE t (i tsvector, j tsvector);
CREATE INDEX t_idx ON t USING gist (i tsvector_ops (siglen = '1800'), j
tsvector_ops);
INSERT INTO t SELECT (SELECT string_agg(g::text, ' ')::tsvector FROM
generate_series(1, 200) g), 'x'::tsvector FROM generate_series(1, 4);

INSERT INTO t SELECT (SELECT string_agg(g::text, ' ')::tsvector FROM
generate_series(1, 200) g) || 'a', 'x'::tsvector;

being executed under Valgrind, triggers:
==00:00:00:57.066 1678143== Conditional jump or move depends on
uninitialised value(s)
==00:00:00:57.066 1678143== at 0x48CBC35: gtsvector_picksplit
(tsgistidx.c:685)
==00:00:00:57.066 1678143== by 0x493208E: FunctionCall2Coll (fmgr.c:1163)
==00:00:00:57.066 1678143== by 0x41409A3: gistUserPicksplit
(gistsplit.c:433)
==00:00:00:57.066 1678143== by 0x414127B: gistSplitByKey
(gistsplit.c:697)
==00:00:00:57.066 1678143== by 0x4141504: gistSplitByKey
(gistsplit.c:747)
==00:00:00:57.066 1678143== by 0x4134199: gistSplit (gist.c:1483)
==00:00:00:57.066 1678143== by 0x4131D58: gistplacetopage (gist.c:316)
==00:00:00:57.066 1678143== by 0x4133D6D: gistinserttuples (gist.c:1310)
==00:00:00:57.066 1678143== by 0x4133CC7: gistinserttuple (gist.c:1263)
==00:00:00:57.066 1678143== by 0x413310D: gistdoinsert (gist.c:892)
==00:00:00:57.067 1678143== by 0x41319E7: gistinsert (gist.c:191)
==00:00:00:57.067 1678143== by 0x4184D62: index_insert (indexam.c:231)
==00:00:00:57.067 1678143==

Reproduced on REL_14_STABLE .. master.