message: return empty list of trackers if torrent handle is invalid
diff: @@ -3,6 +3,7 @@ package com.frostwire.jlibtorrent;
  import com.frostwire.jlibtorrent.swig.*;
  import java.util.ArrayList;
  +import java.util.Collections;
  import java.util.List;
  /**
  @@ -881,6 +882,9 @@ public final class TorrentHandle {
  * @return the list of trackers
  */
  public List<AnnounceEntry> trackers() {
  + if (!th.is_valid()) {
  + return Collections.emptyList();
  + }
  return TorrentInfo.trackers(th.trackers());
  }
  