Fix runtime panic in imagevolume CanSupport method

The following tests are failing right now:

- ci-kubernetes-e2e-ec2-alpha-enabled-default
- ci-kubernetes-e2e-gci-gce-alpha-enabled-default

Because of:

```
goroutine 347 [running]:
k8s.io/apimachinery/pkg/util/runtime.logPanic({0x33092b0, 0x4d6ed00}, {0x296a7e0, 0x4c20c10})
        k8s.io/apimachinery/pkg/util/runtime/runtime.go:107 +0xbc
k8s.io/apimachinery/pkg/util/runtime.handleCrash({0x33092b0, 0x4d6ed00}, {0x296a7e0, 0x4c20c10}, {0x4d6ed00, 0x0, 0x1000000004400a5?})
        k8s.io/apimachinery/pkg/util/runtime/runtime.go:82 +0x5e
k8s.io/apimachinery/pkg/util/runtime.HandleCrash({0x0, 0x0, 0xc000517be8?})
        k8s.io/apimachinery/pkg/util/runtime/runtime.go:59 +0x108
panic({0x296a7e0?, 0x4c20c10?})
        runtime/panic.go:770 +0x132
k8s.io/kubernetes/pkg/volume/image.(*imagePlugin).CanSupport(0xc00183d140?, 0xc0006a2600?)
        k8s.io/kubernetes/pkg/volume/image/image.go:52 +0x3
k8s.io/kubernetes/pkg/volume.(*VolumePluginMgr).FindPluginBySpec(0xc0008a1388, 0xc000f7ddb8)
        k8s.io/kubernetes/pkg/volume/plugins.go:637 +0x208
k8s.io/kubernetes/pkg/kubelet/volumemanager/cache.(*desiredStateOfWorld).AddPodToVolume(0xc000517bc0, {0xc000e94a50, 0x24}, 0xc00172b208, 0xc000f7ddb8, {0xc0017892a0, 0xe}, {0xc000a4d6ec, 0x3}, {0xc000978af0, ...})
        k8s.io/kubernetes/pkg/kubelet/volumemanager/cache/desired_state_of_world.go:270 +0xf2
k8s.io/kubernetes/pkg/kubelet/volumemanager/populator.(*desiredStateOfWorldPopulator).processPodVolumes(0xc0003e6700, 0xc00172b208, 0xc00183ddd8)
        k8s.io/kubernetes/pkg/kubelet/volumemanager/populator/desired_state_of_world_populator.go:319 +0x685
k8s.io/kubernetes/pkg/kubelet/volumemanager/populator.(*desiredStateOfWorldPopulator).findAndAddNewPods(0xc0003e6700)
        k8s.io/kubernetes/pkg/kubelet/volumemanager/populator/desired_state_of_world_populator.go:204 +0x2dc
k8s.io/kubernetes/pkg/kubelet/volumemanager/populator.(*desiredStateOfWorldPopulator).populatorLoop(0xc0003e6700)
        k8s.io/kubernetes/pkg/kubelet/volumemanager/populator/desired_state_of_world_populator.go:173 +0x18
k8s.io/apimachinery/pkg/util/wait.BackoffUntil.func1(0xc000905eb0?)
        k8s.io/apimachinery/pkg/util/wait/backoff.go:226 +0x33
k8s.io/apimachinery/pkg/util/wait.BackoffUntil(0xc00183df70, {0x32d7340, 0xc000a7be60}, 0x1, 0xc0000b2660)
        k8s.io/apimachinery/pkg/util/wait/backoff.go:227 +0xaf
k8s.io/apimachinery/pkg/util/wait.JitterUntil(0xc000f8bf70, 0x5f5e100, 0x0, 0x1, 0xc0000b2660)
        k8s.io/apimachinery/pkg/util/wait/backoff.go:204 +0x7f
k8s.io/apimachinery/pkg/util/wait.Until(...)
        k8s.io/apimachinery/pkg/util/wait/backoff.go:161
k8s.io/kubernetes/pkg/kubelet/volumemanager/populator.(*desiredStateOfWorldPopulator).Run(0xc0003e6700, {0x32e3228, 0xc000b3faa0}, 0xc0000b2660)
        k8s.io/kubernetes/pkg/kubelet/volumemanager/populator/desired_state_of_world_populator.go:158 +0x1a5
created by k8s.io/kubernetes/pkg/kubelet/volumemanager.(*volumeManager).Run in goroutine 335
        k8s.io/kubernetes/pkg/kubelet/volumemanager/volume_manager.go:286 +0x14f
```

Fixes kubernetes#126317

Signed-off-by: Sascha Grunert <[email protected]>

Author: saschagrunert

pkg/volume/image/image.go

@@ -49,7 +49,7 @@ func (o *imagePlugin) GetPluginName() string                           { return
 func (o *imagePlugin) GetVolumeName(spec *volume.Spec) (string, error) { return o.spec.Name(), nil }
 
 func (o *imagePlugin) CanSupport(spec *volume.Spec) bool {
-	return spec.Volume.Image != nil
+	return spec != nil && spec.Volume != nil && spec.Volume.Image != nil
 }
 
 func (o *imagePlugin) NewMounter(spec *volume.Spec, podRef *v1.Pod, opts volume.VolumeOptions) (volume.Mounter, error) {