Compilation fails with MSVC

```
MSVC complains that these functions do not return a value from all paths. 
Adding an unreachable  return after asserting fixes the problem. Patch attached.
```

Original issue reported on code.google.com by `trentfro...@gmail.com` on 26 Jul 2015 at 8:28

Attachments:
- [poly2tri_msvc.diff](https://storage.googleapis.com/google-code-attachments/poly2tri/issue-108/comment-0/poly2tri_msvc.diff)
