Follow-up fixes for client-side slots implementation (ppy#37868)

Fell out of full-stack testing with
ppy/osu-server-spectator#513.

- **Fix missing property copy in multiplayer client**
Would cause the participant count limit to not update on the multiplayer
match screen.
  

- **Fix hard crash when user is kicked from a room with slots active**
The kicked user is unsubscribed from receiving room state updates before
their slot is vacated, which then would lead this code to attempt to
look the local, kicked user via the unvacated slot and thus fail because
`client.Room.Users` does *not* contain the user anymore.
  
This is a bit of a dicey change but I think it's less dicey than to try
to wiggle ordering server-side.

Author: Bartłomiej Dach

osu.Game/Online/Multiplayer/MultiplayerClient.cs

@@ -1011,6 +1011,7 @@ private void updateLocalRoomSettings(MultiplayerRoomSettings settings)
             APIRoom.AutoStartDuration = Room.Settings.AutoStartDuration;
             APIRoom.CurrentPlaylistItem = APIRoom.Playlist.Single(item => item.ID == settings.PlaylistItemId);
             APIRoom.AutoSkip = Room.Settings.AutoSkip;
+            APIRoom.MaxParticipants = Room.Settings.MaxParticipants;
 
             SettingsChanged?.Invoke(settings);
             RoomUpdated?.Invoke();

osu.Game/Screens/OnlinePlay/Multiplayer/Participants/ParticipantsList.cs

@@ -63,7 +63,8 @@ private void updateState()
 
                 for (byte i = 0; i < slotUserIds.Length; ++i)
                 {
-                    var participant = slotUserIds[i] == null ? Slot.Empty(i) : Slot.FromUser(client.Room.Users.Single(u => u.UserID == slotUserIds[i]));
+                    var user = slotUserIds[i] != null ? client.Room.Users.SingleOrDefault(u => u.UserID == slotUserIds[i]) : null;
+                    var participant = user == null ? Slot.Empty(i) : Slot.FromUser(client.Room.Users.Single(u => u.UserID == slotUserIds[i]));
 
                     if (i >= slots.Count)
                         slots.Add(participant);