[secp256k1] Make sure we get the requested number of bytes from /dev/urandom

Summary: Backport PR499 from practicalswift

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: sickpig, tomtomtom7, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1258