st/scroll-view: Use clutter_actor_get_effect() to get fade effect

Storing the pointer to the fade effect only works as long as we control all the code that can modify the effects of an actor, but this is not the case in the presence of extensions. Apparently some extension manages to remove the effect, which leads to fade_effect pointing to some invalid address by the time dispose is called, leading to a crash.

This instead uses clutter_actor_get_effect() to get the effect when it is needed to avoid crashing in these situations.

Closes: #6521 (closed)