Fix get torrent from magnet problem

Fix this problem:

    Traceback (most recent call last):
      File "/home/pouwelse/GITHUB/tribler/Tribler/Core/APIImplementation/ThreadPool.py",
    line 207, in run
        cmd(*args)
      File "/home/pouwelse/GITHUB/tribler/Tribler/Core/Libtorrent/LibtorrentMgr.py",
    line 391, in <lambda>
        self.trsession.uch.perform_usercallback(lambda cb=callback,
    mi=deepcopy(metainfo): cb(mi))
      File "/home/pouwelse/GITHUB/tribler/Tribler/Core/TorrentDef.py",
    line 158, in metainfo_retrieved
        callback(tdef)
      File "/home/pouwelse/GITHUB/tribler/Tribler/Core/RemoteTorrentHandler.py",
    line 786, in __torrentdef_retrieved
        infohash = tdef.get_infohash()
    AttributeError: 'NoneType' object has no attribute 'get_infohash'