Fix exceptions raised when parsing my codebase.

These were the two errors:
```
...
File "cldoc/cldoc/comment.py", line 377, in extract_loop
    if not CommentsDatabase.cldoc_instrre.match(cleaned) is None:
TypeError: expected string or buffer
```
and
```
File "cldoc/cldoc/nodes/method.py", line 43, in override
    bases = list(self.parent.bases)
AttributeError: 'Union' object has no attribute 'bases'
```