gpython: Can not retrieve a function attribute value

## Expected
``` python
>>> def f():
... f.x = 3
... return f.x
...
>>> f()
3
>>>
```

## Actual
```python
>>> def f():
... f.x = 3
... return f.x
...
>>> f()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 FIXME line of source goes here
 File "<stdin>", line 3, in f
 FIXME line of source goes here
SystemError: 'nil Dict in function'
```