Auto merge of #884 - Aaron1011:fix/linux-getrandom, r=RalfJung

Auto merge of #884 - Aaron1011:fix/linux-getrandom, r=RalfJung

Allowing passing a null pointer to getrandom() when length is 0

The Linux kernel will handle a null pointer passed to 'getrandom'
without error, as long as the length is also 0. The `getrandom` crate
relies on this behavior: https://github.com/rust-random/getrandom/blob/ab44edf3c7af721a00e22648b6c811ccb559ba81/src/linux_android.rs#L26

Since it works fine on the actual kernel (and should continue to, due to
the kernel's backwards-compatibility guarantees), Miri should support it
as well.