pgsql: Prevent underflow in KeepLogSeg().

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 c655077639.  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

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/c98b06e2f8655818e83a5a26ef93cc31c357614c

Modified Files
--------------
src/backend/access/transam/xlog.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)