Fix start WAL filename for concurrent backups from standby

On a standby, ThisTimelineID is always 0, so we would generate a
filename in timeline 0 even for other timelines. Instead, use starttli
which we have retreived from the controlfile.

Report by: Francesco Canovai in bug #14230
Author: Marco Nenciarini
Reviewed by: Michael Paquier and Amit Kapila

src/backend/access/transam/xlog.c

@@ -9974,7 +9974,7 @@ do_pg_start_backup(const char *backupidstr, bool fast, TimeLineID *starttli_p,
 		} while (!gotUniqueStartpoint);
 
 		XLByteToSeg(startpoint, _logSegNo);
-		XLogFileName(xlogfilename, ThisTimeLineID, _logSegNo);
+		XLogFileName(xlogfilename, starttli, _logSegNo);
 
 		/*
 		 * Construct tablespace_map file