icons: Ensure positive dimensions

Scaling can result in a zero dimension if the ratio between the dimensions of the original image is too big. See apps/ristretto#116 (closed) for an example of such an image. Opening this image in ristretto causes this warning in xfwm4:

gdk_pixbuf_scale_simple: assertion 'dest_width > 0' failed

Signed-off-by: Gaël Bonithon gael@xfce.org