Failure in __str__() before any output.

When calling str() on a spawn class object before it has any command output, the ``__str__()`` impl. attempts to truncate buffer output while the value of ``self.before`` is still None, raising:

    TypeError: 'NoneType' object has no attribute '__getitem__'
Python does now have a ternary expression:

```python
self.buffer[-100:] if self.buffer else self.buffer
```

Or maybe it should just be set to an empty string in the constructor, so that it's always a string.