refactor: Fix unreachable code in init arg checks (part 2)

Summary:
Follow-up for rABC6c84fca7a26eee3f5f031f02610d4528f7ef8276
which lost the changes for an unknown reason.

Backport of core PR19131.

Test Plan:
With Werror enabled:

ninja all check-all

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8658