effects/thumbnailaside: Fix crash in clientArea()

clientArea() was changed to forbid "-1" screen ids, but it seems like
the thumbnail aside effect can pass it to refer to the active screen.

This change makes the thumbnail aside effect handle "-1" screen id case
explicitly. It will be also useful for EffectScreen transition.

BUG: 443166


(cherry picked from commit 21da7b7e)