[backport#15986] Factor out checksum checking from descriptor parsing

Summary:
https://github.com/bitcoin/bitcoin/pull/15986/commits/104b3a5069c937383e6f88f2f3fb804ef61b208f

Partial backport of Core PR15986

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6622