zmq: Fix implicit-integer-sign-change

Summary:
uint256::begin() returns unsigned data, so there is no reason to make it signed.

Fix that and remove the sanitizer suppression.

This is a backport of core#24218

Depends on D12817

Test Plan:
With UBSAN
ninja && ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12818