SerialNumberMap should decrease current counter if the item exist

public int get(T t) {
  if (t == null) {
    return 0;
  }
  Integer sn = t2i.get(t);
  if (sn == null) {
    sn = current.getAndIncrement();
    if (sn > max) {
      current.getAndDecrement();
      throw new IllegalStateException(name + ": serial number map is full");
    }
    Integer old = t2i.putIfAbsent(t, sn);
    if (old != null) {
      // here: if the old is not null, we should decrease the current value.
      return old;
    }
    i2t.put(sn, t);
  }
  return sn;
}