assumeutxo: Check snapshot base block is not marked invalid

Summary:
Co-authored-by: Alfonso Roman Zubeldia <alfonsoromanz24@gmail.com>

This is a partial backport of core#30267
https://github.com/bitcoin/bitcoin/pull/30267/commits/80315c011863d69e7785673283e4c9033fbcd5ac

Depends on D18055

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18056