Prevent underflow in KeepLogSeg().

The call to XLogGetReplicationSlotMinimumLSN() might return a
greater LSN than the one given to the function.  Subsequent segment
number calculations might then underflow, which could result in
unexpected behavior when removing or recyling WAL files.  This was
introduced with max_slot_wal_keep_size in c655077.  To fix, skip
the block of code for replication slots if the LSN is greater.

Reported-by: Xu Xingwang
Author: Kyotaro Horiguchi
Reviewed-by: Junwang Zhao
Discussion: https://postgr.es/m/17903-4288d439dee856c6%40postgresql.org
Backpatch-through: 13

Author: nathan-bossart

src/backend/access/transam/xlog.c

@@ -7459,7 +7459,7 @@ KeepLogSeg(XLogRecPtr recptr, XLogSegNo *logSegNo)
 	 * max_slot_wal_keep_size.
 	 */
 	keep = XLogGetReplicationSlotMinimumLSN();
-	if (keep != InvalidXLogRecPtr)
+	if (keep != InvalidXLogRecPtr && keep < recptr)
 	{
 		XLByteToSeg(keep, segno, wal_segment_size);