prevent underflow in KeepLogSeg()

When Who What
2023-04-27 21:48:35 Nathan Bossart (bossartn) Closed in commitfest 2023-07 with status: Committed
2023-04-24 19:21:15 Nathan Bossart (bossartn) Changed authors to Kyotaro Horiguchi (horiguti)
2023-04-24 19:21:15 Nathan Bossart (bossartn) Changed committer to bossartn
2023-04-24 19:21:15 Nathan Bossart (bossartn) Changed targetversion to stable
2023-04-24 19:20:54 Nathan Bossart (bossartn) Attached mail thread 17903-4288d439dee856c6@postgresql.org
2023-04-24 19:20:54 Nathan Bossart (bossartn) Created patch record