Avoid triggering undefined behaviour (std::memset(nullptr, 0, 0)) if an invalid string is passed to DecodeSecret(...)

Summary:
Avoid triggering undefined behaviour (std::memset(nullptr, 0, 0)) if an invalid string is passed to DecodeSecret(...).

Backport of Bitcoin Core PR14242
https://github.com/bitcoin/bitcoin/pull/14242

Test Plan:

make check

Reviewers: Fabien, #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4017