Fix for traceback caused by non-standard openTypeOS2WidthClass value.

The traceback was the following:

Traceback (most recent call last):
  File "<string>", line 20, in <module>
  File "/robofab/Lib/robofab/objects/objectsFL.py", line 1001, in writeUFO
    writer.writeInfo(self.info)
  File "/robofab/Lib/robofab/ufoLib.py", line 423, in writeInfo
    value = getattr(info, attr)
  File "/robofab/Lib/robofab/objects/objectsBase.py", line 2954, in __getattr__
    value = self._environmentGetAttr(attr)
  File "/robofab/Lib/robofab/objects/objectsFL.py", line 2911, in _environmentGetAttr
    return method()
  File "/robofab/Lib/robofab/objects/objectsFL.py", line 2945, in _get_styleMapStyleName
    return _styleMapStyleName_fromFL[self._object.font_style]
KeyError: 0