bug: fix segfault when EnableServiceLinks is nil

When upgrading to 1.13, pods that were created prior to the upgrade have
no pod.Spec.EnableServiceLinks set. This causes a segfault and prevents
the pod from ever starting.

Check and set to the default if nil.

Fixes kubernetes#71749

pkg/kubelet/kubelet_pods.go

@@ -533,6 +533,10 @@ func (kl *Kubelet) getServiceEnvVarMap(ns string, enableServiceLinks bool) (map[
 // Make the environment variables for a pod in the given namespace.
 func (kl *Kubelet) makeEnvironmentVariables(pod *v1.Pod, container *v1.Container, podIP string) ([]kubecontainer.EnvVar, error) {
 	var result []kubecontainer.EnvVar
+	enableServiceLinks := v1.DefaultEnableServiceLinks
+	if pod.Spec.EnableServiceLinks != nil {
+		enableServiceLinks = *pod.Spec.EnableServiceLinks
+	}
 	// Note:  These are added to the docker Config, but are not included in the checksum computed
 	// by kubecontainer.HashContainer(...).  That way, we can still determine whether an
 	// v1.Container is already running by its hash. (We don't want to restart a container just
@@ -542,7 +546,7 @@ func (kl *Kubelet) makeEnvironmentVariables(pod *v1.Pod, container *v1.Container
 	// To avoid this users can: (1) wait between starting a service and starting; or (2) detect
 	// missing service env var and exit and be restarted; or (3) use DNS instead of env vars
 	// and keep trying to resolve the DNS name of the service (recommended).
-	serviceEnv, err := kl.getServiceEnvVarMap(pod.Namespace, *pod.Spec.EnableServiceLinks)
+	serviceEnv, err := kl.getServiceEnvVarMap(pod.Namespace, enableServiceLinks)
 	if err != nil {
 		return result, err
 	}