Fix Clang Static Analyzer warnings

Summary:
This is a backport of Core PR12963

• Fix dead stores. Values were stored but never read. Limit scope.
• Add assertion to guide static analyzers. Clang Static Analyzer needs this guidance.

Depends on D3364

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3377