Ensure room is locked when mutating users

osu.Game/Online/RealtimeMultiplayer/MultiplayerRoom.cs

@@ -9,16 +9,33 @@ namespace osu.Game.Online.RealtimeMultiplayer
     [Serializable]
     public class MultiplayerRoom
     {
+        private object writeLock = new object();
+
         public long RoomID { get; set; }
 
         public MultiplayerRoomState State { get; set; }
 
-        public IReadOnlyList<MultiplayerRoomUser> Users => users;
-
         private List<MultiplayerRoomUser> users = new List<MultiplayerRoomUser>();
 
-        public void Join(int user) => users.Add(new MultiplayerRoomUser(user));
+        public IReadOnlyList<MultiplayerRoomUser> Users
+        {
+            get
+            {
+                lock (writeLock)
+                    return users.ToArray();
+            }
+        }
 
-        public void Leave(int user) => users.RemoveAll(u => u.UserID == user);
+        public void Join(int user)
+        {
+            lock (writeLock)
+                users.Add(new MultiplayerRoomUser(user));
+        }
+
+        public void Leave(int user)
+        {
+            lock (writeLock)
+                users.RemoveAll(u => u.UserID == user);
+        }
     }
 }