restructure character validity check to be more readable and correct

[endigma]

• drops the 2-armed switch statement in favor of if
• adds check that the key doesn't contain non-crockford32 runes

I'd also recommend adding test coverage for the validity fail branch of Decode() and perhaps running a full validation instead of only checking length. It looks like it would likely panic or fail with undefined behavior as the error result from crock32.Decode() is disregarded on L126.

[jackspirou]

Thanks @endigma - LGTM!