Merge #10961: Improve readability of DecodeBase58Check(...)

Summary:
c6a995e Improve readability of DecodeBase58Check(...) (practicalswift)

Pull request description:

Use the more readable form ...

```c++
&vchRet[vchRet.size() - 4]
```

... instead of ...

```c++
&v.end()[-n]
```

Has the added benefit of eliminating a spurious static analyzer warning about improper use of negative values.

Tree-SHA512: 5895310c189e9322082c28f34342ff9a6c238e2cae3f204521111c8a7981bc555af60b42de082c91608c1125dfc244a65c4faf929249a067a51435e2be74cb39

Backport of Core PR10961
https://github.com/bitcoin/bitcoin/pull/10961/files

Test Plan: make check

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3228