Fix compiler warnings emitted when compiling under stock OpenBSD 6.3

Summary:
Fix compiler warnings emitted when compiling under stock OpenBSD 6.3 (OpenBSD clang version 5.0.1, based on LLVM 5.0.1):

Backport of Bitcoin Core PR13294
https://github.com/bitcoin/bitcoin/pull/13294

Test Plan:
make check-all is OK on my platform (Linux 5.2.15-200.fc30.x86_64 x86_64)

I don't have an OpenBSD 6.3 to test it.
It doesn't seem that the changes are going to have problems on it.

Reviewers: Fabien, #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4141