Fix some more copies in loops

Summary: These copies where catched by clang-7 but not by clang-6.

Test Plan:
With clang-7:

ninja check

There should be no -Wrange-loop-analysis warning.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3807