ulib: fix flaky proof
ulib: fix flaky proof

seems to succeed only with about 13% of random seeds, it just happened
that 0 was one of them.

I've never seen it fail with this one

FStarLang/FStar,A-Manning/FStar,FStarLang/FStar,A-Manning/FStar,FStarLang/FStar,A-Manning/FStar