Use FRC::randbytes instead of reading >32 bytes from RNG
There was only one place in the codebase where we're directly reading >32 bytes from
the RNG. One possibility would be to make the built-in RNG support large reads, but
using FastRandomContext lets us reuse code better.
There is no change in behavior here, because the FastRandomContext constructor
uses GetRandBytes internally.
Partial backport of Core PR14955
Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc
Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D4387