Fix: guarantee to reserve an IP address from IPAM pool for k8s POD

Returns port number as zero in case pod has null spec value.
Also return null group bucket when we have zero port number returned.

Change-Id: Iad6f307e950e263732db5f6349c83a60a194cb23
diff --git a/apps/k8s-networking/app/src/main/java/org/onosproject/k8snetworking/util/K8sNetworkingUtil.java b/apps/k8s-networking/app/src/main/java/org/onosproject/k8snetworking/util/K8sNetworkingUtil.java
index 045b300..7d20b65 100644
--- a/apps/k8s-networking/app/src/main/java/org/onosproject/k8snetworking/util/K8sNetworkingUtil.java
+++ b/apps/k8s-networking/app/src/main/java/org/onosproject/k8snetworking/util/K8sNetworkingUtil.java
@@ -492,6 +492,11 @@
      *         return 0 if there is no port number mapped with the given port name
      */
     public static int portNumberByName(Pod pod, String portName) {
+
+        if (pod == null || pod.getSpec() == null) {
+            return 0;
+        }
+
         for (Container container : pod.getSpec().getContainers()) {
             for (ContainerPort cp : container.getPorts()) {
                 if (cp.getName() != null && cp.getName().equals(portName)) {