prevent negative complete/incomplete numbers

Fixes webtorrent#65
Fixes webtorrent#72

Author: feross

lib/swarm.js

@@ -19,8 +19,6 @@ Swarm.prototype.announce = function (params, cb) {
   if (self[fn]) {
     self[fn](params, peer) // process event
 
-    if (params.left === 0 && peer) peer.complete = true
-
     cb(null, {
       complete: self.complete,
       incomplete: self.incomplete,
@@ -86,6 +84,12 @@ Swarm.prototype._onAnnounce_update = function (params, peer) {
     debug('unexpected `update` event from peer that is not in swarm')
     return this._onAnnounce_started(params, peer) // treat as a start
   }
+
+  if (!peer.complete && params.left === 0) {
+    this.complete += 1
+    this.incomplete -= 1
+    peer.complete = true
+  }
 }
 
 // TODO: randomize the peers that are given out