Merge #10530: Fix invalid instantiation and possibly unsafe accesses of array in class base_uint<BITS>

Summary:
e5c6168 Fix instantiation and array accesses in class base_uint<BITS> (Pavlos Antoniou)

Tree-SHA512: e4d39510d776c5ae8814cd5fb5c5d183cd8da937e339bff95caff68a84492fbec68bf513c5a6267446a564d39093e0c7fc703c645b511caab80f7baf7955b804

Backport of Core PR10530
https://github.com/bitcoin/bitcoin/pull/10530

Test Plan:

make check
test_runner.py

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

Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3569