operator: fix panic for non existing CEPs

```
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x118 pc=0x1dcb219]
goroutine 161 [running]:
main.enableUnmanagedKubeDNSController.func1(0x28aa1a0, 0xc00069e880, 0x3d3d0a0, 0xd9fc43ebeec50347)
	/go/src/github.com/cilium/cilium/operator/k8s_pod_controller.go:67 +0xa69
github.com/cilium/cilium/pkg/controller.(*Controller).runController(0xc000789a00)
	/go/src/github.com/cilium/cilium/pkg/controller/controller.go:205 +0xa2a
created by github.com/cilium/cilium/pkg/controller.(*Manager).updateController
	/go/src/github.com/cilium/cilium/pkg/controller/manager.go:120 +0xb09
```

Fixes: 26e7451 ("operator: use watcher to track unmanaged pods")
Reported-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: André Martins <andre@cilium.io>

operator/k8s_pod_controller.go

@@ -61,11 +61,14 @@ func enableUnmanagedKubeDNSController() {
 						continue
 					}
 					cep, exists, err := watchers.HasCE(pod.Namespace, pod.Name)
+					if err != nil {
+						log.WithError(err).Errorf("unexpected error when getting CiliumEndpoint %s/%s", pod.Namespace, pod.Name)
+						continue
+					}
 					podID := fmt.Sprintf("%s/%s", pod.Namespace, pod.Name)
-					switch {
-					case err == nil:
+					if exists {
 						log.Debugf("Found kube-dns pod %s with identity %d", podID, cep.Status.ID)
-					case !exists:
+					} else {
 						log.Debugf("Found unmanaged kube-dns pod %s", podID)
 						if startTime := pod.Status.StartTime; startTime != nil {
 							if age := time.Since((*startTime).Time); age > unmanagedKubeDnsMinimalAge {
@@ -88,8 +91,6 @@ func enableUnmanagedKubeDNSController() {
 
 							}
 						}
-					default:
-						return err
 					}
 				}