pgsql: Fix stale idle flag when IO workers exit.

Fix stale idle flag when IO workers exit.

Otherwise we could choose a worker that has exited and crash while
trying to wake it up.

Back-patch to 18.

Reported-by: Tomas Vondra <tomas@vondra.me>
Reported-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/t5aqjhkj6xdkido535pds7fk5z4finoxra4zypefjqnlieevbg%40357aaf6u525j

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/20b8b5dab983a11b8c8b5ba95f1d6f073ad26300

Modified Files
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src/backend/storage/aio/method_worker.c | 2 ++
1 file changed, 2 insertions(+)