Use secure allocator for RNG state

Summary:
Partial backport of Core PR14955
https://github.com/bitcoin/bitcoin/pull/14955/commits/f2e60ca98530e0a865ff6c6fd3c5633aec11a515

Depends on D4399

Test Plan:

make check

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

Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4400