fix cmdDel crash when use pod annotation "v1.multus-cni.io/default-network"

Crash happens in code line `conf.Delegates[0] = delegate` in function TryLoadPodDelegates,
because len(conf.Delegates) is 0.

Signed-off-by: Huanle Han <hanhuanle@caicloud.io>

multus/multus.go

@@ -413,6 +413,15 @@ func cmdDel(args *skel.CmdArgs, exec invoke.Exec, kubeClient k8s.KubeClient) err
 		return logging.Errorf("Multus: Err in getting k8s args: %v", err)
 	}
 
+	if in.ClusterNetwork != "" {
+		err = k8s.GetDefaultNetworks(k8sArgs, in, kubeClient)
+		if err != nil {
+			return logging.Errorf("Multus: Failed to get clusterNetwork/defaultNetworks: %v", err)
+		}
+		// First delegate is always the master plugin
+		in.Delegates[0].MasterPlugin = true
+	}
+
 	numK8sDelegates, kc, err := k8s.TryLoadPodDelegates(k8sArgs, in, kubeClient)
 	if err != nil {
 		return err