RE: Slot's restart_lsn may point to removed WAL segment after hard restart unexpectedly

Hi,

After commit ca307d5, I noticed another crash when testing
some other logical replication features.

The server with max_replication_slots set to 0 would crash when executing CHECKPOINT.

TRAP: failed Assert("ReplicationSlotCtl != NULL"), File: "slot.c", Line: 1162, PID: 577315
postgres: checkpointer (ExceptionalCondition+0x9e)[0xc046cb]
postgres: checkpointer (ReplicationSlotsComputeRequiredLSN+0x30)[0x99697f]
postgres: checkpointer (CheckPointReplicationSlots+0x191)[0x997dc1]
postgres: checkpointer [0x597b1b]
postgres: checkpointer (CreateCheckPoint+0x6d1)[0x59729e]
postgres: checkpointer (CheckpointerMain+0x559)[0x93ee79]
postgres: checkpointer (postmaster_child_launch+0x15f)[0x940311]
postgres: checkpointer [0x9468b0]
postgres: checkpointer (PostmasterMain+0x1258)[0x9434f8]
postgres: checkpointer (main+0x2fe)[0x7f5f9c]
/lib64/libc.so.6(__libc_start_main+0xe5)[0x7f7585f81d85]
postgres: checkpointer (_start+0x2e)[0x4958ee]

I think it is trying to access the replication slots when the shared memory
for them was not allocated.

Best Regards,
Hou zj