Fix the race condition in ReplicationSlotAcquire().

Fix the race condition in ReplicationSlotAcquire().

After commit f41d8468dd, a process could acquire and use a replication
slot that had just been invalidated, leading to failures while accessing
WAL.

To ensure that we don't accidentally start using invalid slots, we must
perform the invalidation check after acquiring the slot or under the
spinlock where we associate the slot with a particular process. We choose
the earlier method to keep the code simple.

Reported-by: Hou Zhijie <houzj.fnst@fujitsu.com>
Author: Nisha Moond <nisha.moond412@gmail.com>
Reviewed-by: Hou Zhijie <houzj.fnst@fujitsu.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Discussion: https://postgr.es/m/CABdArM7J-LbGoMPGUPiFiLOyB_TZ5+YaZb=HMES0mQqzVTn8Gg@mail.gmail.com