Be pedantic with Window's icon indexes.

Documentation for SHFILEINFO says iIcon is an index, so lets
initialize defaultFolderIIcon with -1 so it doesn't clash
with some icon that might exist at index 0.

Change-Id: Ic16538ee62e5433f3cdcceee19eb5d8d18d55c1e
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>