Merge #13722: trivial: Replace CPubKey::operator[] with CPubKey::vch where possible

Summary:
6755569840 trivial: Replace CPubKey::operator[] with CPubKey::vch where possible (Nikolay Mitev)

Pull request description:

Use variable name instead of calling operator[] through &(*this)[0]

Tree-SHA512: 7054ffda0fa33fb45d4d9f3b29698643f02fd1421d78d5197a0881f2c368dc410647fd2e1a6feb8048e30f8ab8bc2fa8749bf42b9ccbe42c30de8ff80ac45274

Backport of Core PR13722
https://github.com/bitcoin/bitcoin/pull/13722/

Test Plan:

make check

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4248