Make lsn argument of walrcv_create_slot() optional

Some callers are not using it, so it's wasteful to have to specify it.
Reviewed-by: Masahiko Sawada <masahiko.sawada@2ndquadrant.com>
Discussion: https://www.postgresql.org/message-id/CA+fd4k4BcYrYucNfTnK-CQX3+jsG+PRPEhHAUSo-W4P0Lec57A@mail.gmail.com

src/backend/commands/subscriptioncmds.c

@@ -428,7 +428,6 @@ CreateSubscription(CreateSubscriptionStmt *stmt, bool isTopLevel)
 	 */
 	if (connect)
 	{
-		XLogRecPtr	lsn;
 		char	   *err;
 		WalReceiverConn *wrconn;
 		List	   *tables;
@@ -479,7 +478,7 @@ CreateSubscription(CreateSubscriptionStmt *stmt, bool isTopLevel)
 				Assert(slotname);
 
 				walrcv_create_slot(wrconn, slotname, false,
-								   CRS_NOEXPORT_SNAPSHOT, &lsn);
+								   CRS_NOEXPORT_SNAPSHOT, NULL);
 				ereport(NOTICE,
 						(errmsg("created replication slot \"%s\" on publisher",
 								slotname)));

src/backend/replication/libpqwalreceiver/libpqwalreceiver.c

@@ -844,8 +844,10 @@ libpqrcv_create_slot(WalReceiverConn *conn, const char *slotname,
 						slotname, pchomp(PQerrorMessage(conn->streamConn)))));
 	}
 
+	if (lsn)
 		*lsn = DatumGetLSN(DirectFunctionCall1Coll(pg_lsn_in, InvalidOid,
 												   CStringGetDatum(PQgetvalue(res, 0, 1))));
+
 	if (!PQgetisnull(res, 0, 2))
 		snapshot = pstrdup(PQgetvalue(res, 0, 2));
 	else