Segfault in custom JSON decoder implementation

[rtfeldman]

roc check and roc build both succeed, but running it segfaults.

Repro: https://gist.github.com/travisstaloch/7add1274a085f1e6cba59ca93d991d9c Discussion: https://roc.zulipchat.com/#narrow/stream/231634-beginners/topic/json.20parser/near/302777765