Segmentation fault: 11

Hi, I have the segfault even with the primitive example:
```
const parser = require('parser-xml2json')

const xml = '<population></population>'
parser.parseString(xml, (err, res) => console.log(err, res))
```

```
$ node -v
v8.2.1
$ rustc --version
rustc 1.21.0-nightly (599be0d18 2017-07-26)
```