Merge #14734: fix an undefined behavior in uint::SetHex

Summary:
0f459d868d85053f1cc066ea9099793f88cbd655 fix an undefined behavior in uint::SetHex (Kaz Wesley)

Pull request description:

Decrementing psz beyond the beginning of the string is UB, even though
the out-of-bounds pointer is never dereferenced.

I don't think any clang sanitizer covers this, so I don't see any way a test could catch the original behavior.

ACKs for top commit:

promag:
  utACK 0f459d8.
l2a5b1:
  utACK 0f459d868d85053f1cc066ea9099793f88cbd655

Tree-SHA512: 388223254ea6e955f643d2ebdf74d15a3d494e9f0597d9f05987ebb708d7a1cc06ce64bd25d447d75b5f5561bdae9630dcf25adb7bd75f7a382298b95d127162

Backport of Core PR14734

Test Plan: ninja check

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6844