Fix panic when k3sURLEnvIndex is -1

[moio]

What

Fixes a panic condition - first found in terraform-provider-k3d acceptance tests.

Why

Symptom is:

panic: runtime error: index out of range [-1]

goroutine 1713 [running]:
github.com/k3d-io/k3d/v5/pkg/client.NodeAddToCluster(...)
	/home/runner/go/pkg/mod/github.com/k3d-io/k3d/v5@v5.4.9/pkg/client/node.go:200 +0x1ed0

I believe there is a logic mistake in the code pointed by the error above, and this PR attempts to address it.

Specifically, k3sURLEnvIndex can be -1 when node.Env[k3sURLEnvIndex] is evaluated - specifically when k3sURLEnvFound is false. So introduce a condition to guard the lookup on the slice in that case.

Additionally, since k3sURLEnvFound == false implies checkK3SURLIsActive(...) == false (there can't be an active URL if none was found in the first place), remove dead code in the first branch of the if statement and rework the condition to work equivalently.

Implications

Should only fix this particular panic and not cause any adverse side effects.

@all-contributors please add @moio for code

[moio]

cc @4p00rv as code here was introduced in #1190 - I'd appreciate a look to double check I did not accidentally alter the intended original meaning.

[4p00rv]

@moio That code was unnecessary. Thanks for simplifying. Looks good from my end :)

[moio]

Is there anything more I could do to have this merged?

[iwilltry42]

Thank you very much for this change @moio and thanks @4p00rv for checking :)

[iwilltry42]

OK, I'm a little lost and don't know what happened here. With a base branch update this was closed 🤔

[moio]

Well, I see the commit in the main branch, so I am happy 😉

Thanks!