Raise error when streaming from a removed torrent

lib/file-stream.js

@@ -56,6 +56,8 @@ FileStream.prototype._notify = function () {
   if (self._notifying) return
   self._notifying = true
 
+  if (self._torrent.destroyed) return self._destroy(new Error('Torrent removed'))
+
   var p = self._piece
   self._torrent.store.get(p, function (err, buffer) {
     self._notifying = false