[button] Add a temporary solution to the animation/dispose race
[button] Add a temporary solution to the animation/dispose race

Sometimes the mx_animation_completed callback gets fired after the button has
already been finalised. This temporary solution checks that the button is
really a button before dereferencing the private data.

pwithnall/mx,clutter-project/mx,clutter-project/mx,jonnylamb/mx,pwithnall/mx,jonnylamb/mx,jonnylamb/mx,pwithnall/mx,pwithnall/mx,clutter-project/mx,jonnylamb/mx,clutter-project/mx