Fix a problem when completing a torrent (etorrent_peer_control)

[jlouis]

Here is the crash context we are trying to fix inside etorrent_peer_control:

  {{badarg,[{etorrent_peerstate,choked,2,
                              [{file,"src/etorrent_peerstate.erl"},
                               {line,120}]},
          {etorrent_peer_control,handle_message,2,
                                 [{file,"src/etorrent_peer_control.erl"},
                                  {line,535}]},
          {etorrent_peer_control,handle_cast,2,
                                 [{file,"src/etorrent_peer_control.erl"},
                                  {line,319}]},
          {gen_server,handle_msg,5,[{file,"gen_server.erl"},{line,607}]},
          {proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,227}]}]},
 [{gen_server,terminate,6,[{file,"gen_server.erl"},{line,747}]},
  {proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,227}]}]}