event: fixed subscribtions to stopped event mux

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.

event/event_test.go

@@ -25,6 +25,14 @@ import (
 
 type testEvent int
 
+func TestSubCloseUnsub(t *testing.T) {
+	// the point of this test is **not** to panic
+	var mux TypeMux
+	mux.Stop()
+	sub := mux.Subscribe(int(0))
+	sub.Unsubscribe()
+}
+
 func TestSub(t *testing.T) {
 	mux := new(TypeMux)
 	defer mux.Stop()