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