join_panic

```golang

 kubeadm join phase preflight -v 10
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x13b78f9]

goroutine 1 [running]:
k8s.io/kubernetes/cmd/kubeadm/app/cmd.newJoinData(0xc000451400, 0xc0000a2d20, 0x18e8b60, 0xc000094008, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
	cmd/kubeadm/app/cmd/join.go:347 +0xf9
k8s.io/kubernetes/cmd/kubeadm/app/cmd.NewCmdJoin.func2(0xc000451400, 0x0, 0x0, 0x0, 0x0)
	cmd/kubeadm/app/cmd/join.go:218 +0x73
k8s.io/kubernetes/cmd/kubeadm/app/cmd/phases/workflow.(*Runner).InitData(0xc00014ebd0, 0xc0003d3c80, 0x0, 0x0, 0xc000533110)
	cmd/kubeadm/app/cmd/phases/workflow/runner.go:184 +0x67
k8s.io/kubernetes/cmd/kubeadm/app/cmd/phases/workflow.(*Runner).Run(0xc00014ebd0, 0xc0003ebcd0, 0x551634)
	cmd/kubeadm/app/cmd/phases/workflow/runner.go:204 +0x72
k8s.io/kubernetes/cmd/kubeadm/app/cmd/phases/workflow.(*Runner).BindToCommand.func1.1(0xc000451400, 0xc0001e4500, 0x0, 0x2)
	cmd/kubeadm/app/cmd/phases/workflow/runner.go:350 +0xc4
k8s.io/kubernetes/vendor/github.com/spf13/cobra.(*Command).execute(0xc000451400, 0xc0001e44a0, 0x2, 0x2, 0xc000451400, 0xc0001e44a0)
	vendor/github.com/spf13/cobra/command.go:760 +0x2cc
k8s.io/kubernetes/vendor/github.com/spf13/cobra.(*Command).ExecuteC(0xc00038e280, 0xc0000d4a00, 0xc00038f180, 0xc0003e97b0)
	vendor/github.com/spf13/cobra/command.go:846 +0x2fd
k8s.io/kubernetes/vendor/github.com/spf13/cobra.(*Command).Execute(0xc00038e280, 0xc000094000, 0x18e8b60)
	vendor/github.com/spf13/cobra/command.go:794 +0x2b
k8s.io/kubernetes/cmd/kubeadm/app.Run(0xc000096180, 0x18b)
	cmd/kubeadm/app/kubeadm.go:48 +0x202
main.main()
	cmd/kubeadm/kubeadm.go:29 +0x33

```