simplify handling unsupported numbers
simplify handling unsupported numbers

Removes the usage of ++need, and therefore any possibility of
accidentally crashing, by performing the check of unsupported numbers
before anything else.

Idea originally suggested by @philipcmonk
https://github.com/urbit/arvo/pull/1053#issuecomment-460814835

ngzax/urbit,ngzax/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,joemfb/arvo,jfranklin9000/urbit,belisarius222/arvo,urbit/urbit,jfranklin9000/urbit,belisarius222/arvo,urbit/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,jfranklin9000/urbit,urbit/urbit,jfranklin9000/urbit,urbit/urbit,jfranklin9000/urbit,joemfb/arvo,urbit/urbit,jfranklin9000/urbit,jfranklin9000/urbit,ngzax/urbit