refactor: Pass reference to last header, not pointer

Summary:
It is never a nullptr, otherwise an assertion would fire in
UpdatePeerStateForReceivedHeaders.

Passing a reference makes the code easier to read and less brittle.

This is a backport of core#26378

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16817