randbelow_from_randbits no ValueError, kenral crashing #56

r = SHA256(1)
r.randbelow_from_randbits(-1)

r = SHA256(1)
r.randbelow_from_randbits(-1)

Both of these function calls cause kernel errors in jupyterhub, and unlike in the docstring for randbelow_from_randbits, no ValueError is raised if n==0.

I suggest assert n > 0 is added