libweston: fix crash when a client binds to a destroyed output

When the head is already destroyed, its global resource still can be
accessible by the client, which leads to a UAF crash.

This sets the head's global resource's user data to null before the
head is destroyed, and when the `bind` request is being handled but the
object's user data is null, do what we do when the the head's output is
null.

Signed-off-by: Paul Pu <hui.pu@gehealthcare.com>