This can happen if the async is terminated while there is no thread to queue
the APC to (as in the relevant test), or if the client dies before getting the
APC, or before transferring the APC results back to the server.

This also fixes a leak of async objects present since 61abc500. If a process
dies while accept asyncs are pending, the asyncs will be terminated but will
not find a valid thread to queue the APC to, and thus async_set_result() and
the completion callback are never called.
Signed-off-by: Zebediah Figura <z.figura12@gmail.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>