Infinite loop when an invalid callout list follows a list item

```javascript
const asciidoctor = require('asciidoctor')()
asciidoctor.convert(Buffer.from('103a3a0a393e0901', 'hex'))
```
**Version** 2.0.3

Found with [jsfuzz](https://github.com/fuzzitdev/jsfuzz)