Fix potentially starting play when finished

The UserFinishedPlaying event may trigger before the event is subscribed
to by SpectatorScreen. For such cases, an extra check is done to make
sure the user is _actually_ playing.

osu.Game/Screens/Spectate/SpectatorScreen.cs

@@ -127,6 +127,10 @@ namespace osu.Game.Screens.Spectate
                 if (!userMap.ContainsKey(userId))
                     return;
 
+                // The user may have stopped playing.
+                if (!spectatorClient.TryGetPlayingUserState(userId, out _))
+                    return;
+
                 Schedule(() => OnUserStateChanged(userId, state));
 
                 updateGameplayState(userId);