Bug in Sha512Random.split(1)

[gafter]

There is a bug in Sha512Random.split(1) exposed by the following test.

    @Test fun split_One() {
        val k1 = RandomKey()
        val k2 = k1.split(1)[0];
        val k3 = k2.split(1)[0];
    }

The bug is in these lines:

    override fun split(n: Int): List<Sha512Random> {
        if (hasBeenUsed && !permitsReuse)
            throw IllegalStateException("RandomKey instance has already been used.")
        hasBeenUsed = true
        // No need to split if n==1
        if (n == 1) return listOf(this)

The code invalidates this and then returns it as if it is valid.