state: Fix crasher when removing the last key of a transitioning state
state: Fix crasher when removing the last key of a transitioning state

If ClutterState is in the middle of a transition and you remove all the
keys from the target state, the target state will be destroyed without
stopping the animation/unsetting the target state. This caused an invalid
memory access.

heysion/clutter-clone,kerrickstaley/clutter-vala,Distrotech/clutter,collects/clutter,djdeath/clutter,collects/clutter,dlespiau/clutter,spatulasnout/clutter,heysion/clutter-clone,ebassi/clutter,Distrotech/clutter,djdeath/clutter-multithreaded,djdeath/clutter,collects/clutter,spatulasnout/clutter,collects/clutter,djdeath/clutter-android,dlespiau/clutter,djdeath/clutter,jigpu/clutter,jigpu/clutter,GNOME/clutter,nobled/clutter,nobled/clutter,kerrickstaley/clutter-vala,jigpu/clutter,ebassi/clutter,spatulasnout/clutter,heysion/clutter-clone,kerrickstaley/clutter-vala,heysion/clutter-clone,jigpu/clutter,djdeath/clutter,GNOME/clutter,spatulasnout/clutter,collects/clutter,ebassi/clutter,ebassi/clutter,djdeath/clutter-android,djdeath/clutter,nobled/clutter,GNOME/clutter,Distrotech/clutter,spatulasnout/clutter,djdeath/clutter-android,djdeath/clutter,Distrotech/clutter,GNOME/clutter,dlespiau/clutter,heysion/clutter-clone,Distrotech/clutter,jigpu/clutter,djdeath/clutter-multithreaded,djdeath/clutter-android,jigpu/clutter,kerrickstaley/clutter-vala,djdeath/clutter-multithreaded,ebassi/clutter,Distrotech/clutter,djdeath/clutter-multithreaded,GNOME/clutter,djdeath/clutter-android