fix out-of-bounds memory write in key_tests

Summary: Introduced by me in D2348. Thanks to Jason for catching this with ASan.

Test Plan:
make check

I ran ASan-compiled ./src/test/test_bitcoin --run_test=key_tests with and without this patch. Prior to patch it revealed memory error "WRITE of size 32"; afterwards it has no errors.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D2923