Fix clang-tidy error

Summary: See https://build.bitcoinabc.org/viewLog.html?tab=buildLog&logTab=tree&filter=debug&expand=all&buildId=77717&guest=1#footer

Test Plan:

./build-configurations.py build-clang-tidy

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7723