darwin: Eliminate a possible race condition while destroying a surface

Introduced by: c60ffd2840036af1ea6f2b6c6e1e9014bb8e2c34
Signed-off-by: Jeremy Huddleston <jeremyhu@apple.com>
(cherry picked from commit d65bd195ecbd6623b962a3c98725a484ef2791a8)
1 file changed