Fix bug in multixact Oldest*MXactId initialization and access

When Who What
2026-03-18 08:26:26 John Naylor (john.naylor) Closed in commitfest PG19-Final with status: Committed
2026-03-18 08:26:26 John Naylor (john.naylor) Changed committer to heikki
2026-02-24 17:53:54 Yura Sokolov (funny_falcon) Changed targetversion to None
2026-02-24 17:53:40 Yura Sokolov (funny_falcon) Changed targetversion to 17
2026-02-24 17:41:41 Yura Sokolov (funny_falcon) Attached mail thread 7acc94b0-ea82-4657-b1b0-77842cb7a60c@postgrespro.ru
2026-02-24 17:41:41 Yura Sokolov (funny_falcon) Created patch record