pypy seg fault on larger byte arrays

This came up in a pykafka compression issue.

https://github.com/Parsely/pykafka/issues/508

So byte arrays above a certain size causes a seg fault.

```
(pypy) ➜ cat s.py
from uuid import uuid4
import snappy

payload = b''.join([uuid4().bytes for i in range(10)])

c = snappy.compress(payload)
assert snappy.decompress(c) == payload
(pypy) ➜ python s.py
[1] 4587 segmentation fault (core dumped) python s.py
(pypy) ➜ python
Python 2.7.10 (bbd45126bc69, Mar 18 2016, 21:35:08)
[PyPy 5.0.1 with GCC 4.8.4] on linux2
```

while

```
from uuid import uuid4
import snappy

payload = b''.join([uuid4().bytes for i in range(5)])

c = snappy.compress(payload)
assert snappy.decompress(c) == payload
```

passes