Kubectx panicked if the current context is deleted

The current context is `arn:aws:eks:eu-central-1:XXXXXXX:cluster/ls-eks-dev`.

```
$ kubectl config get-contexts
CURRENT NAME CLUSTER AUTHINFO NAMESPACE
* arn:aws:eks:eu-central-1:XXXXXXX:cluster/ls-eks-dev arn:aws:eks:eu-central-1:XXXXXXX:cluster/ls-eks-dev arn:aws:eks:eu-central-1:XXXXXXX:cluster/ls-eks-dev
arn:aws:eks:eu-central-1:XXXXXXX:cluster:cluster/ls-eks-dev-02 arn:aws:eks:eu-central-1:XXXXXXX:cluster/ls-eks-dev-02 arn:aws:eks:eu-central-1:XXXXXXX:cluster/ls-eks-dev-02
```

If I delete it:

```
$ kubectl config delete-context arn:aws:eks:eu-central-1:XXXXXXX::cluster/ls-eks-dev
warning: this removed your active context, use "kubectl config use-context" to select a different one
deleted context arn:aws:eks:eu-central-1:XXXXXXX::cluster/ls-eks-dev from /Users/jerome.lecorvaisier/.kube/config
```

Then I run `kubectx` to switch to another context, then it fails:

```
$ kubectx
? Select kubectl context (Use arrow keys)
thread 'main' panicked at 'current selected context should be in selection', src/libcore/option.rs:823
note: Run with `RUST_BACKTRACE=1` for a backtrace.
```

So, it might be useful if we can still switch to another context instead of being stuck with this.

```
$ kubectl config get-contexts
CURRENT NAME CLUSTER AUTHINFO NAMESPACE
arn:aws:eks:eu-central-1:XXXXXXX:cluster/ls-eks-dev-02 arn:aws:eks:eu-central-1:XXXXXXXcluster/ls-eks-dev-02 arn:aws:eks:eu-central-1:XXXXXXX:cluster/ls-eks-dev-02
```

And if I try to set another context using `kubectl`, `kubectx` still get stuck.

```
$ kubectl config set-context arn:aws:eks:eu-central-1:XXXXXXX:cluster/ls-eks-dev-02
Context "arn:aws:eks:eu-central-1:XXXXXXX:cluster/ls-eks-dev-02" modified.

$ kubectx
? Select kubectl context (Use arrow keys)
thread 'main' panicked at 'current selected context should be in selection', src/libcore/option.rs:823
note: Run with `RUST_BACKTRACE=1` for a backtrace.
```