Fix attlen in RowDescription of BASE_BACKUP response

Should be 8 for int8, not -1.

Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/7e4fdbdc-699c-4cd0-115d-fb78a957fc22@enterprisedb.com

src/backend/replication/basebackup_copy.c

@@ -361,7 +361,7 @@ SendXlogRecPtrResult(XLogRecPtr ptr, TimeLineID tli)
 	 * would not be wide enough for this, as TimeLineID is unsigned.
 	 */
 	pq_sendint32(&buf, INT8OID);	/* type oid */
-	pq_sendint16(&buf, -1);
+	pq_sendint16(&buf, 8);
 	pq_sendint32(&buf, 0);
 	pq_sendint16(&buf, 0);
 	pq_endmessage(&buf);