Fix parser crash in ++lung:vast.
Fix parser crash in ++lung:vast.

Closes #554.

ngzax/urbit,urbit/urbit,eglaysher/arvo,ohAitch/arvo,joemfb/arvo,jfranklin9000/urbit,urbit/urbit,urbit/urbit,jfranklin9000/urbit,cgyarvin/arvo,belisarius222/arvo,jfranklin9000/urbit,jfranklin9000/urbit,cgyarvin/arvo,jfranklin9000/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,ngzax/urbit,urbit/urbit,ngzax/urbit,belisarius222/arvo,jfranklin9000/urbit,urbit/urbit,urbit/urbit,Fang-/arvo,joemfb/arvo,jfranklin9000/urbit,ngzax/urbit,ngzax/urbit,solri/arvo,ohAitch/arvo