pgsql: Fix CheckPointReplicationSlots() with max_replication_slots == 0

Fix CheckPointReplicationSlots() with max_replication_slots == 0

ca307d5cec90 made CheckPointReplicationSlots() unconditionally call
ReplicationSlotsComputeRequiredLSN(). It causes an assertion trap when
max_replication_slots equals 0. This commit makes
CheckPointReplicationSlots() call ReplicationSlotsComputeRequiredLSN() only
when at least one slot gets its last_saved_restart_lsn updated. That avoids
an assert trap and also saves some cycles when no one slot has
last_saved_restart_lsn updated.

Based on ideas from Dilip Kumar <dilipbalaut(at)gmail(dot)com> and
Hayato Kuroda <kuroda(dot)hayato(at)fujitsu(dot)com>.

Reported-by: Zhijie Hou <houzj(dot)fnst(at)fujitsu(dot)com>
Discussion: https://postgr.es/m/OS0PR01MB5716BB506AF934376FF3A8BB947BA%40OS0PR01MB5716.jpnprd01.prod.outlook.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/7195c804bd12f47a9f1b2df9c2e1794bb04c5987

Modified Files
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src/backend/replication/slot.c | 15 ++++++++++++---
1 file changed, 12 insertions(+), 3 deletions(-)