Automatic merge from submit-queue
Add assert.NotNil for test case
I hardcode the `DefaultInterfaceName` from `eth0` to `eth-k8sdefault` at release 1.2.0, in order to test my CNI plugins. When running the test, it panics and prints wrongly formatted messages as below.
In the test case `TestBuildSummary`, `containerInfoV2ToNetworkStats` will return `nil` if `DefaultInterfaceName` is not `eth0`. So maybe we should add `assert.NotNil` to the test case.
```
ok k8s.io/kubernetes/pkg/kubelet/server 0.591s
W0523 03:25:28.257074 2257 summary.go:311] Missing default interface "eth-k8sdefault" for s%!(EXTRA string=node:FooNode)
W0523 03:25:28.257322 2257 summary.go:311] Missing default interface "eth-k8sdefault" for s%!(EXTRA string=pod:test0_pod1)
W0523 03:25:28.257361 2257 summary.go:311] Missing default interface "eth-k8sdefault" for s%!(EXTRA string=pod:test0_pod0)
W0523 03:25:28.257419 2257 summary.go:311] Missing default interface "eth-k8sdefault" for s%!(EXTRA string=pod:test2_pod0)
--- FAIL: TestBuildSummary (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x0 pc=0x471817]
goroutine 16 [running]:
testing.func·006()
/usr/src/go/src/testing/testing.go:441 +0x181
k8s.io/kubernetes/pkg/kubelet/server/stats.checkNetworkStats(0xc20806d3b0, 0x140bbc0, 0x4, 0x0, 0x0)
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/kubelet/server/stats/summary_test.go:296 +0xc07
k8s.io/kubernetes/pkg/kubelet/server/stats.TestBuildSummary(0xc20806d3b0)
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/kubelet/server/stats/summary_test.go:124 +0x11d2
testing.tRunner(0xc20806d3b0, 0x1e43180)
/usr/src/go/src/testing/testing.go:447 +0xbf
created by testing.RunTests
/usr/src/go/src/testing/testing.go:555 +0xa8b
```
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