wIconChangeImageFile don't update without file
The function wIconChangeImageFile mustn't update the icon if
no file is set.
This is a bug. To reproduce it:
1. Right click on dock icon. Select "Settings"
2. Erase the Icon Image contents (set empty)
3. Click "OK" -> Exit, no changes... no?
4. Right click on the same dock icon. Select "Settings"
5. Erase the Icon Image contents (set empty)
6. Click "OK" -> Crash
If wmaker don't crash, watch the file ~/GNUstep/Defaults/WMWindowAttributes,
the default icon ("*" has trash in the name). Probably something is not fine
at wIconUpdate.