validation: default initialize and guard chainman members
Summary:
This is a partial backport of core#22564
https://github.com/bitcoin/bitcoin/pull/22564/commits/6e747e80e7094df0b5bee1eed57e57e82015d0ee
Test Plan:
With clang and DEBUG
ninja all check-all
Reviewers: #bitcoin_abc, sdulfari
Reviewed By: #bitcoin_abc, sdulfari
Differential Revision: https://reviews.bitcoinabc.org/D13086