add a check as to whether icon_info is None

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1036273

```
Traceback (most recent call last):
  File "/usr/lib/cinnamon-desktop-editor/cinnamon-desktop-editor.py", line 104, in pick_icon
    icon_info_fn = icon_info.get_filename()
AttributeError: 'NoneType' object has no attribute 'get_filename'
```