pgsql: Fix one-off bug causing missing commit timestamps for subtransac

Fix one-off bug causing missing commit timestamps for subtransactions

The logic in charge of writing commit timestamps (enabled with
track_commit_timestamp) for subtransactions had a one-bug bug,
where it would be possible that commit timestamps go missing for the
last subtransaction committed.

While on it, simplify a bit the iteration logic in the loop writing the
commit timestamps, as per suggestions from Kyotaro Horiguchi and Tom
Lane, so as some variable initializations are not part of the loop
itself.

Issue introduced in 73c986a.

Analyzed-by: Alex Kingsborough
Author: Alex Kingsborough, Kyotaro Horiguchi
Discussion: https://postgr.es/m/73A66172-4050-4F2A-B7F1-13508EDA2144@amazon.com
Backpatch-through: 10

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b5f634116e8f3e264cc06c053bd7b4e25a615665

Modified Files
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src/backend/access/transam/commit_ts.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)