client: Consume file descriptors destined for zombie proxies

We need to close file descriptors sent to zombie proxies to avoid leaking
them, and perhaps more importantly, to prevent them from being dispatched
in events on other objects (since they would previously be left in the
buffer and potentially fed to following events destined for live proxies)

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