wallet: Fix non-determinism in ParseHDKeypath(...). Avoid using an uninitialized variable in path calculation.

Summary:

• wallet: Add error handling. Check return value of ParseUInt32(...) in ParseHDKeypath(...).
• wallet: Add tests for ParseHDKeypath(...)

This is a backport of Core PR13712

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4500