Require CBlockIndex::GetUndoPos() to hold mutex cs_main

Summary:
This is a partial backport of core#22932
https://github.com/bitcoin/bitcoin/pull/22932/commits/572393448b4d32f91b92edc84b4200ab52d62422

Depends on D13032

Test Plan:
With clang and DEBUG:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13033