Fix Leaderboard IndexOutOfBoundsException

src/de/steamwar/lobby/util/Leaderboard.java

@@ -37,6 +37,7 @@ public class Leaderboard {
         entities.forEach(REntity::die);
         entities.clear();
         List<LeaderboardEntry> leaderboard = getLeaderboard();
+        if (leaderboard.isEmpty()) return;
         long bestTime = leaderboard.get(0).time;
         for (int i = 0; i <= best; i++) {
             if (best - i >= leaderboard.size()) continue;