nil pointer dereference in ValidateClusterUpdate when old cluster is nil

[YooLCD]

/kind bug

1. What kops version are you running?

Latest master (confirmed in source).

2. What Kubernetes version are you running?

N/A

3. What cloud provider are you using?

N/A

4. What commands did you run? What is the simplest way to reproduce this issue?

validation.ValidateClusterUpdate(cluster, nil, nil, nil)

5. What happened after the commands executed?

panic: runtime error: invalid memory address or nil pointer dereference

k8s.io/kops/pkg/apis/kops/validation.ValidateClusterUpdate(...)
        pkg/apis/kops/validation/cluster.go:39

6. What did you expect to happen?

ValidateClusterUpdate should handle a nil old cluster gracefully and return validation errors for the new cluster only.

7. Please provide your cluster manifest.

N/A

8. Please run the commands with most verbose logging by adding the -v 10 flag.

N/A

9. Anything else do we need to know?

Root cause: cluster.go:39 iterates over old.Spec.EtcdClusters without a nil guard:

for _, etcdCluster := range old.Spec.EtcdClusters {

Fix: add an early return before the etcd cluster validation block:

if old == nil {
        return allErrs
}

If this is accepted, I am happy to submit a PR with the fix.