Fix -Wshadow warnings

Summary: These warnings have been introduced in D4891.

Test Plan:
With GCC (for some reason clang -tested with 9.0.1- doesn't report the
warning):

cmake -GNinja .. -DCMAKE_CXX_FLAGS="-Werror=shadow"
ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4919