Add error message in case of unexpected situation

To troubleshooting #481, adds error message for further analysis.

multus/multus.go

@@ -362,6 +362,9 @@ func delegateAdd(exec invoke.Exec, kubeClient *k8s.ClientInfo, pod *v1.Pod, ifNa
 		} else {
 			kubeClient.Eventf(pod, v1.EventTypeNormal, "AddedInterface", "Add %s %v", rt.IfName, ips)
 		}
+	} else {
+		// for further debug https://github.com/intel/multus-cni/issues/481
+		logging.Errorf("delegateAdd: pod nil pointer: namespace: %s, name: %s, container id: %s, pod: %v", rt.Args[1][1], rt.Args[2][1], rt.Args[3][1], pod)
 	}
 
 	return result, nil