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 Reported-by: Andres Freund Discussion: https://postgr.es/m/t5aqjhkj6xdkido535pds7fk5z4finoxra4zypefjqnlieevbg%40357aaf

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 Reported-by: Andres Freund Discussion: https://postgr.es/m/t5aqjhkj6xdkido535pds7fk5z4finoxra4zypefjqnlieevbg%40357aaf