refactor: Pass block reference instead of pointer to PeerManagerImpl::BlockRequested

Summary:
This allows to remove an assert and at the same time make it more obvious that the block is never nullptr.

This is a backport of core#22221

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12714