client: Add wl_object_is_zombie() helper function

Add a helper function which determines whether or not an object is a
zombie.

[daniels: Extracted from Derek's bespoke-zombie patch as an intermediate
          step.]

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
3 files changed