Fix a BF failure caused by commit 76b78721ca.

Fix a BF failure caused by commit 76b78721ca.

The issue occurred because the replication slot was not released in the
slotsync worker when a slot synchronization cycle was skipped. This skip
happened because the required WAL was not received and flushed on the
standby server. As a result, in the next cycle, when attempting to acquire
the slot, an assertion failure was triggered.

Author: Hou Zhijie <houzj.fnst@fujitsu.com>
Discussion: https://postgr.es/m/CAA4eK1KMwYUYy=oAVHu9mam+vX50ixxfhO4_C=kgQC8VCQHEfw@mail.gmail.com