Fix synchronized_standby_slots GUC check hook

Fix synchronized_standby_slots GUC check hook

The validate_sync_standby_slots subroutine requires an LWLock, so it
cannot run in processes without PGPROC; skip it there to avoid a crash.

This replaces the current test for ReplicationSlotCtl being not null,
which appears to be a solution for the same problem but less general.
I also rewrote a related comment that mentioned ReplicationSlotCtl in
StandbySlotsHaveCaughtup.

This code came in with commit bf279ddd1c28; backpatch to 17.

Reported-by: Gabriele Bartolini <gabriele.bartolini@enterprisedb.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-by: Zhijie Hou <houzj.fnst@fujitsu.com>
Discussion: https://postgr.es/m/202411281216.sutbxtr6idnn@alvherre.pgsql