Fix bug when opening single files in ST3

There was a little problem with [this pull request](https://github.com/quarnster/SublimeClang/pull/207) when opening single files in ST3. When no project is opened,  `window.project_file_name()` returns `None` and therefore, `os.path.split` throws 
```python
AttributeError: 'NoneType' object has no attribute 'rfind'
```
Just added a little condition to avoid the plugin to crash.

Anyway, thanks for all the good work! 
I hope I'll be able to take some time to help with your completion project soon.