compositor: Really avoid painting a screen of zero size

Bug: 15852

Previous attempt to fix a crash with a screensize of 0, which can occur
without any output connected was incomplete (commit 3925109d) as a
client might still trigger a repaint by queuing up damages.

Make sure we take this case into account as well.

Signed-off-by: Olivier Fourdan <fourdan@xfce.org>