no more -Wmissing-braces warnings

Summary: self-explanatory

Test Plan: run make with -Wmissing-braces and see no warnings

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2191