perform nextTick work only if work needs to be done (#931)

* perform nextTick work only if work needs to be done, call done() immediately if not.

* check if done() is valid

lib/client.js

@@ -256,18 +256,24 @@ MqttClient.prototype._setupStream = function () {
   })
 
   function nextTickWork () {
-    process.nextTick(work)
+    if (packets.length) {
+      process.nextTick(work)
+    } else {
+      var done = completeParse
+      completeParse = null
+      done()
+    }
   }
 
   function work () {
     var packet = packets.shift()
-    var done = completeParse
 
     if (packet) {
       that._handlePacket(packet, nextTickWork)
     } else {
+      var done = completeParse
       completeParse = null
-      done()
+      if (done) done()
     }
   }