fix(Blockchain): handle downloadPeer being unset while requesting headers

A race condition could previously happen where we try to request headers but error since there is no downloadPeer.

Author: mappum

lib/blockchain.js

@@ -343,6 +343,7 @@ Blockchain.prototype._getHeaders = function (opts) {
   opts = opts || {}
   this.getLocator(opts.from || this.tip.hash, function (err, locator) {
     if (err) throw err
+    if (!self.downloadPeer) return
     var message = self.downloadPeer.messages.GetHeaders({
       starts: locator
     })