Automatic merge from submit-queue
Fix nil pointer issue when making mounts for container
When rebooting one of the nodes in my colleague's cluster, two panics were discovered:
```
E1216 04:07:00.193058 2394 runtime.go:52] Recovered from panic: "invalid memory address or nil pointer dereference" (runtime error: invalid memory address or nil pointer dereference)
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/util/runtime/runtime.go:58
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/util/runtime/runtime.go:51
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/util/runtime/runtime.go:41
/usr/local/go/src/runtime/asm_amd64.s:472
/usr/local/go/src/runtime/panic.go:443
/usr/local/go/src/runtime/panic.go:62
/usr/local/go/src/runtime/sigpanic_unix.go:24
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/kubelet/kubelet.go:1313
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/kubelet/kubelet.go:1473
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/kubelet/dockertools/docker_manager.go:1495
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/kubelet/dockertools/docker_manager.go:2125
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/kubelet/dockertools/docker_manager.go:2093
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/kubelet/kubelet.go:1971
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/kubelet/kubelet.go:530
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/kubelet/pod_workers.go:171
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/kubelet/pod_workers.go:154
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/kubelet/pod_workers.go:215
/usr/local/go/src/runtime/asm_amd64.s:1998
E1216 04:07:00.275030 2394 runtime.go:52] Recovered from panic: "invalid memory address or nil pointer dereference" (runtime error: invalid memory address or nil pointer dereference)
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/util/runtime/runtime.go:58
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/util/runtime/runtime.go:51
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/util/runtime/runtime.go:41
/usr/local/go/src/runtime/asm_amd64.s:472
/usr/local/go/src/runtime/panic.go:443
/usr/local/go/src/runtime/panic.go:62
/usr/local/go/src/runtime/sigpanic_unix.go:24
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/kubelet/server/stats/volume_stat_caculator.go:98
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/kubelet/server/stats/volume_stat_caculator.go:63
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/util/wait/wait.go:86
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/util/wait/wait.go:87
/usr/local/go/src/runtime/asm_amd64.s:1998
```
kubectl version
```
Client Version: version.Info{Major:"1", Minor:"3", GitVersion:"v1.3.8", GitCommit:"693ef591120267007be359f97191a6253e0e4fb5", GitTreeState:"clean", BuildDate:"2016-09-28T03:03:21Z", GoVersion:"go1.6.2", Compiler:"gc", Platform:"linux/amd64"}
Server Version: version.Info{Major:"1", Minor:"3", GitVersion:"v1.3.8", GitCommit:"693ef591120267007be359f97191a6253e0e4fb5", GitTreeState:"clean", BuildDate:"2016-09-28T02:52:25Z", GoVersion:"go1.6.2", Compiler:"gc", Platform:"linux/amd64"}
```
The second panic had already been fixed by #33616 and #34251. Not sure what caused the first nil pointer issue and whether it has been fixed yet in the master branch. Just fix it by ignoring the nil pointer when making mounts.
cc @jingxu97 @yujuhong
c1ebba0ae2