Fix negative leecher count issue

Fixes webtorrent#65

Author: feross

lib/swarm.js

@@ -47,7 +47,7 @@ Swarm.prototype._onAnnounce_started = function (params, peer) {
   if (params.left === 0) this.complete += 1
   else this.incomplete += 1
   peer = this.peers[params.addr || params.peer_id] = {
-    complete: false,
+    complete: params.left === 0,
     ip: params.ip, // only http+udp
     peerId: params.peer_id, // as hex
     port: params.port, // only http+udp