Catch the case where we cannot find any nodes.

It's possible to fall through the loops above with node still nil.  This
catches this and reports an error.

Found this working on #27819.

pkg/dns/dns.go

@@ -659,6 +659,10 @@ func (kd *KubeDNS) getClusterZone() (string, error) {
 		}
 	}
 
+	if node == nil {
+		return "", fmt.Errorf("Could not find any nodes")
+	}
+
 	zone, ok := node.Annotations[unversioned.LabelZoneFailureDomain]
 	if !ok || zone == "" {
 		return "", fmt.Errorf("unknown cluster zone")