segfault on surrogate unicode

```python
Python 3.6.1 (default, Mar 23 2017, 02:34:11)
[GCC 4.9.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import json
>>> import rapidjson
>>> rapidjson.__version__
'0.2.4'
>>> foo = {"foo": "\ud80d"}
>>> json.dumps(foo)
'{"foo": "\\ud80d"}'
>>> rapidjson.dumps(foo)
Segmentation fault (core dumped)
```