This fixes an issue where the following would lead to a panic due to a
channel being closed twice:
* Start mux
* Stop mux
* Sub to mux
* Unsub
This is fixed by setting the subscriptions status to closed resulting in
the Unsubscribe to ignore the request when called.
(cherry picked from commit 7c1f74713e273d1e4f1982466eb6fd96e55e8c4d)
7e5c49cafa