actor: Ensure allocation adjustment is safe against zero sizes
actor: Ensure allocation adjustment is safe against zero sizes

We already copy with negative end results, but there's no point in doing
the work in the first place.

jigpu/clutter,GNOME/clutter,Distrotech/clutter,jigpu/clutter,GNOME/clutter,GNOME/clutter,jigpu/clutter,GNOME/clutter,Distrotech/clutter,Distrotech/clutter,Distrotech/clutter,jigpu/clutter,GNOME/clutter,jigpu/clutter,Distrotech/clutter,jigpu/clutter,Distrotech/clutter