Fix UBSan error: Shifting 64 bit type by 64 is undefined behaviour

[DanielG]

Shifting a type by >=sizeof(type)*8 is undefined behaviour in C. Since
t->prng.buf is overwritten on the next iteration anyways this thankfully
isn't a real bug here.

Fixes #52

[silentbicycle]

Being able to take more than 64 bits here could be a sign of a deeper issue. Thanks for reporting this.

I probably won't get to reviewing this closely for a bit because of the holidays, but when I do I'll retarget the PR for the develop branch.

[dcreager]

This just bit me too! I think the fix is correct — there's no attempt to take more than 64 bits, which you're right would be a deeper issue. It's just that if you take exactly 64 bits, then there's no need to shift anything in prng.buf, since you've just consumed it all, and will replace it with a new random uint64_t the next time through the loop. The if guard in this patch checks exactly that.