Initialize pieces on constructor

Many prototype functions and properties use it and if it hasn't been initialized it will throw error as it's undefined.
webtorrent · DiegoRBaquero · Dec 9, 2015 · Dec 9, 2015 · Dec 9, 2015 · Dec 9, 2015
commit efa410dc279bff80f5ab6ffcbcc87279e3737908
diff --git a/lib/torrent.js b/lib/torrent.js
@@ -81,8 +81,9 @@ function Torrent (torrentId, opts) {
   this.numBlockedPeers = 0
   this.files = null
   this.done = false
-
+  
   this._amInterested = false
+  this.pieces = []
   this._selections = []
   this._critical = []