Work around GCC -Wmaybe-uninitialized false positive

Summary:
This works around a GCC bug:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80635

Test Plan:
With an affected GCC version:

ninja all check

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8630