font.path could be None

test if the path is None before getting the dirname

see https://github.com/typesupply/feaPyFoFum/blob/master/Lib/feaPyFoFum/feaPyFoFum.py#L39

``` python
relativePath = None
if font.path is not None:
    relativePath = os.path.dirname(font.path)
```
