p2p: avoid calculating onion address checksum when version is not 3

Summary:
There is no need to calculate the full checksum for an Tor v3 onion address if the version byte is not the expected one.

This is a backport of core#20405

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10733