...also update comments to remove mention of ::ChainActive()
From: https://github.com/bitcoin/bitcoin/pull/20750#discussion_r579400663
> Also, what about passing a const reference instead of a pointer? I
> know this is only theoretical, but previously if the tip was nullptr,
> then Height() evaluated to -1, now it evaluates to UB
Note: in Bitcoin ABC, there was already a check for `active_chain_tip == nullptr`, but only for `nMedianPastTime. A few lines above we accessed the pointer to get the `nHeight` without checking first (potential UB introduced in D11191).
This is a backport of [[https://github.com/bitcoin/bitcoin/pull/21055 | core#21055]] [1/16]
https://github.com/bitcoin/bitcoin/pull/21055/commits/5e4af773809415b71a10e3120cc44854d61c4c19