Bugfix: randbytes should seed when needed (non reachable issue)

Summary: This is a backport of Core PR14624 : https://github.com/bitcoin/bitcoin/pull/14624/commits/8d98d426116f0178612f14d1874d331042c4c4b7

Test Plan:

make check

Reviewers: #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3446