Fix an NPE when logging out while in a game.

The fix for #124 apparently introduced another bug wherein logging out while playing in a game caused an NPE while ensuring that the user was also not spectating the game. Hold onto that value a little bit longer to make sure we can call that method as well, even though it shouldn't need to be called at that point (but an NPE is worse).

src/main/java/net/socialgamer/cah/data/User.java

@@ -294,8 +294,9 @@ public class User {
    */
   public void noLongerValid() {
     if (currentGame != null) {
-      currentGame.removePlayer(this);
-      currentGame.removeSpectator(this);
+      final Game game = currentGame;
+      game.removePlayer(this);
+      game.removeSpectator(this);
     }
     valid = false;
   }