Address nits from per-utxo change

Summary:

• Add belt-and-suspenders in DisconnectBlock

These extra variables were previously checked before the move to per-txout database.

• Comment clarifications in coins.cpp

This is a backport of core PR10558

Test Plan:

make check

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1789