Level.String panics if Level == InvalidLevel

[clementauger]

hi,

the method LeveL.String panics when Level==InvalidLevel.

https://github.com/apex/log/blob/master/levels.go#L44 https://github.com/apex/log/blob/master/levels.go#L17

I suggest to add a statement before the slice access so that this pseudo value does not appear in the levelNames / levelStrings variables.

It could return an empty string or something like invalid

[tj]

👍 sounds good to me