Derek Foreman authored Aug 18, 2023 and Marius Vlad committed Sep 15, 2023

This is a trap - it's not harmful to let the animation complete - it is
harmful to call weston_shell_utils_curtain_destroy() in mid animation. It
results in immediately completing the animation, which attempts to destroy
the curtain a second time.

Fixes a crash when (on a multi-output system) the display is disconnected
during the fade out, then reconnected before the fade-in starts.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>