storage/stdout: nil-check Cpu, Network, and Memory stats

[locker95]

Problem

With -storage_driver=stdout, cAdvisor panics:

panic: runtime error: invalid memory address or nil pointer dereference
...
stdout.(*stdoutStorage).containerStatsToValues
    stdout.go:133

Line 133 is stats.Network.RxBytes. After the model change, Cpu, Network, and Memory on ContainerStats are pointers and may be nil for some containers. The stdout driver still treated them as values.

Fix

• Guard CPU and network fields in containerStatsToValues
• Guard memory fields in memoryStatsToValues
• Add a unit test that used to panic (Cpu set, Network/Memory nil)

Test

cd cmd && go test ./internal/storage/stdout/ -count=1

Fixes #3911

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[locker95]

cla: yes

[locker95]

I have signed the Google CLA with this GitHub account (locker95) and authorized email 862469039@qq.com.

cla: yes

Please recheck.