Remove erroneous assertion from pg_copy_logical_replication_slot().

If restart_lsn of logical replication slot gets behind more than
max_slot_wal_keep_size from the current LSN, the logical replication slot
would be invalidated and its restart_lsn is reset to an invalid LSN.
If this logical replication slot with an invalid restart_lsn was specified as
the source slot in pg_copy_logical_replication_slot(), the function caused
the assertion failure unexpectedly.

This assertion was added because restart_lsn should not be invalid before.
But in v13, it can be invalid thanks to max_slot_wal_keep_size. So since this
assertion is no longer useful, this commit removes it.

This commit also changes the errcode in the error message that
pg_copy_logical_replication_slot() emits when the slot with an invalid
restart_lsn is specified, to more appropriate one.

Back-patch to v13 where max_slot_wal_keep_size was added and
the assertion was no longer valid.

Author: Fujii Masao
Reviewed-by: Alvaro Herrera, Kyotaro Horiguchi
Discussion: https://postgr.es/m/f91de4fb-a7ab-b90e-8132-74796e049d51@oss.nttdata.com

src/backend/replication/slotfuncs.c

@@ -755,12 +755,9 @@ copy_replication_slot(FunctionCallInfo fcinfo, bool logical_slot)
 
 	/* Copying non-reserved slot doesn't make sense */
 	if (XLogRecPtrIsInvalid(src_restart_lsn))
-	{
-		Assert(!logical_slot);
 		ereport(ERROR,
-				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
 				 errmsg("cannot copy a replication slot that doesn't reserve WAL")));
-	}
 
 	/* Overwrite params from optional arguments */
 	if (PG_NARGS() >= 3)