[bitcoinsuite-core] Fix a potential out of bounds access during address decoding

Summary: The code expects the checksum to be present and will cause the slice to access out of bounds otherwise. This diff changes to logic to verify the checksum first so this can't happen anymore.

Test Plan:

ninja check-crate-bitcoinsuite-core

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18121