Fix stats error when disconnecting on first join

src/de/anura/core/API/Level.java

@@ -73,6 +73,9 @@ public final class Level {
     }
 
     public void update(Player P) {
+        if (!anuraLevel.containsKey(P)) {
+            return;
+        }
         saveStuff(P);
         try {
             String whereClause = " = (SELECT id FROM players WHERE uuid = '" + P.getUniqueId().toString() + "')";