Gardenctl panics when accessing node that has not joined the cluster

[schrodit]

Describe the bug
Gardenctl panics when trying to ssh into a node that has not yet joined the cluster.

Downloaded id_rsa key
Fetching coreos AMI
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0xb0 pc=0x1dabd58]

goroutine 1 [running]:
github.com/gardener/gardenctl/pkg/cmd.fetchAWSImageIDByInstancePrivateIP(0x7ffeefbff7c6, 0xc, 0xc000601538, 0x1)
	/go/src/pkg/cmd/ssh.go:317 +0x238
github.com/gardener/gardenctl/pkg/cmd.sshToAWSNode(0x7ffeefbff7c6, 0xc, 0xc0000842a0, 0x61)
	/go/src/pkg/cmd/ssh.go:140 +0xb4
github.com/gardener/gardenctl/pkg/cmd.NewSSHCmd.func1(0xc0000e4280, 0xc0002f0090, 0x1, 0x1, 0x0, 0x0)
	/go/src/pkg/cmd/ssh.go:117 +0x595
github.com/spf13/cobra.(*Command).execute(0xc0000e4280, 0xc0002f0040, 0x1, 0x1, 0xc0000e4280, 0xc0002f0040)
	/go/src/vendor/github.com/spf13/cobra/command.go:826 +0x460
github.com/spf13/cobra.(*Command).ExecuteC(0x2c5f980, 0x1d, 0xc000042740, 0x1d)
	/go/src/vendor/github.com/spf13/cobra/command.go:914 +0x2fb
github.com/spf13/cobra.(*Command).Execute(...)
	/go/src/vendor/github.com/spf13/cobra/command.go:864
github.com/gardener/gardenctl/pkg/cmd.Execute()
	/go/src/pkg/cmd/root.go:104 +0x2ae
main.main()
	/go/src/cmd/gardenctl/main.go:20 +0x20

https://github.com/gardener/gardenctl/blob/master/pkg/cmd/ssh.go#L317

The same command is working for gardenctl 0.11.0

To Reproduce
Steps to reproduce the behavior:

1. gardenctl ssh

Expected behavior
SSH into node or error message that states that the node has not joined the cluster and is unable to be accessed.

Gardenctl Version (please complete the following information):

• 0.13.0

[ialidzhikov]

Thanks for reporting!
This should be fixed with #131. If you manage to reproduce with the master branch g/gctl, please reopen.