fix mesos client panic while ExecutorPID is nill

[Komey]

From mesos-go ContainerStatus spec ExecutorPID can be null with *uint32.
This will cause nil pointer panic from line L111

// *
// Container related information that is resolved during container
// setup. The information is sent back to the framework as part of the
// TaskStatus message.
type ContainerStatus struct {
	ContainerID *ContainerID `protobuf:"bytes,4,opt,name=container_id,json=containerId" json:"container_id,omitempty"`
	// This field can be reliably used to identify the container IP address.
	NetworkInfos []NetworkInfo `protobuf:"bytes,1,rep,name=network_infos,json=networkInfos" json:"network_infos"`
	// Information about Linux control group (cgroup).
	CgroupInfo *CgroupInfo `protobuf:"bytes,2,opt,name=cgroup_info,json=cgroupInfo" json:"cgroup_info,omitempty"`
	// Information about Executor PID.
	ExecutorPID *uint32 `protobuf:"varint,3,opt,name=executor_pid,json=executorPid" json:"executor_pid,omitempty"`
}

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[k8s-ci-robot]

Hi @Komey. Thanks for your PR.

I'm waiting for a google member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[bobbypage]

/ok-to-test

[bobbypage]

Thanks for the contribution - please approve the CLA - #3129 (comment)

[Komey]

Thanks for the contribution - please approve the CLA - #3129 (comment)

https://github.com/google/cadvisor/pull/3129/checks?check_run_id=6858815936

HI @bobbypage CLA has been approved

[bobbypage]

Thanks!

/ok-to-test

[bobbypage]

/retest

[bobbypage]

Thanks!