refactor: fix unread variable warnings

[jmayclin]

Description of changes:

Cppcheck is failing in #4404 because zero initializing variables is causing new unreadVariable warnings to surface. Rather than adding new suppressions, this PR fixes those warnings by pushing the variable declaration down a scope.

This is being done in a separate PR to keep #4404 as an entirely automated PR.

Call-outs:

Note that under any reasonable optimization level this code is expected to be identical. This was spot checked with GodBolt: https://godbolt.org/z/vaYeWecfM.

Testing:

I first gathered all of the unreadVariable warnings from cppcheck. After fixing those, I first reapplied the clang-tidy fixes

run-clang-tidy . -p build/ -use-color -quiet -checks=cppcoreguidelines-init-variables -fix

I then ran cppcheck

cppcheck --enable=style \
    --project=build/compile_commands.json \
    --output-file=report.txt \
    -j $(nproc)

I finally verified that there were 0 unreadVariable warnings in the new report.

[ec2-user]$ cat report.txt | grep unreadVariable | wc -l
0

Current CI and tests should be sufficient to ensure that there was no behavior change.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.