Fails on existential conditional in throw calls

Decaffeinate is crashing on my CoffeeScript input:

```coffee
throw(res.data?.error)
```

[(repl)](http://decaffeinate-project.org/repl/#?evaluate=true&stage=full&code=throw(res.data%3F.error))

I get this error:

```
AddVariableDeclarationsStage failed to parse: Unexpected token (1:6)
```