server: do not send delete_id to a dead client

During client tear-down, all objects are destroyed in id order.
Therefore the display object is destroyed first.

If the destroy listeners of any object destroy another object by calling
wl_resoruce_destroy(), we try to send a delete_id event to the client.
This leads to a segmentation fault without a display object.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>