Fix panic by disallowing multiple room joins (#3149)

Release Notes:

- Fixed panic that could occur when switching channels quickly

crates/collab/src/tests.rs

@@ -40,3 +40,7 @@ fn room_participants(room: &ModelHandle<Room>, cx: &mut TestAppContext) -> RoomP
         RoomParticipants { remote, pending }
     })
 }
+
+fn channel_id(room: &ModelHandle<Room>, cx: &mut TestAppContext) -> Option<u64> {
+    cx.read(|cx| room.read(cx).channel_id())
+}