Fix comment in xlogreader.c

This has been introduced by 709d003f, that has moved readSegNo, readOff
and readPageTLI into a new structure called WALOpenSegment initialized
separately.

Author: Kyotaro Horiguchi
Discussion: https://postgr.es/m/20190926.110809.248342687.horikyota.ntt@gmail.com

src/backend/access/transam/xlogreader.c

@@ -103,8 +103,7 @@ XLogReaderAllocate(int wal_segment_size, const char *waldir,
 	state->read_page = pagereadfunc;
 	/* system_identifier initialized to zeroes above */
 	state->private_data = private_data;
-	/* ReadRecPtr and EndRecPtr initialized to zeroes above */
-	/* readSegNo, readOff, readLen, readPageTLI initialized to zeroes above */
+	/* ReadRecPtr, EndRecPtr and readLen initialized to zeroes above */
 	state->errormsg_buf = palloc_extended(MAX_ERRORMSG_LEN + 1,
 										  MCXT_ALLOC_NO_OOM);
 	if (!state->errormsg_buf)