clutter/actor: Sanity check new allocations
Apparently some shell extensions are setting invalid NaN allocations, leading to weird crashes like gnome-shell#1849 (closed).
Even though an implementation error like this probably deserves a crash, those can be hard to debug since the crash can happen anywhere the allocation is being used later. So let Clutter be the good guy and prevent implementations from setting invalid allocations by sanity-checking the ClutterActorBoxes using g_return_if_fail.