Fixed cb accessing code.

If `outgoing[mid]` doesn't match, then accessing `outgoing[mid].cb`
causes `Cannot read property` error. So added checking code.

Fixed outgoing assignment at `_onConnect` function.

lib/client.js

@@ -793,7 +793,7 @@ MqttClient.prototype.end = function () {
  * @example client.removeOutgoingMessage(client.getLastMessageId());
  */
 MqttClient.prototype.removeOutgoingMessage = function (mid) {
-  var cb = this.outgoing[mid].cb
+  var cb = this.outgoing[mid] ? this.outgoing[mid].cb : null
   delete this.outgoing[mid]
   this.outgoingStore.del({messageId: mid}, function () {
     cb(new Error('Message removed'))
@@ -978,7 +978,7 @@ MqttClient.prototype._storePacket = function (packet, cb, cbStorePut) {
   if (((packet.qos || 0) === 0 && this.queueQoSZero) || packet.cmd !== 'publish') {
     this.queue.push({ packet: packet, cb: cb })
   } else if (packet.qos > 0) {
-    cb = this.outgoing[packet.messageId].cb
+    cb = this.outgoing[packet.messageId] ? this.outgoing[packet.messageId].cb : null
     this.outgoingStore.put(packet, function (err) {
       if (err) {
         return cb && cb(err)
@@ -1193,7 +1193,7 @@ MqttClient.prototype._handleAck = function (packet) {
   var mid = packet.messageId
   var type = packet.cmd
   var response = null
-  var cb = this.outgoing[mid].cb
+  var cb = this.outgoing[mid] ? this.outgoing[mid].cb : null
   var that = this
   var err
 
@@ -1416,14 +1416,17 @@ MqttClient.prototype._onConnect = function (packet) {
 
       // Avoid unnecessary stream read operations when disconnected
       if (!that.disconnecting && !that.reconnectTimer) {
-        cb = that.outgoing[packet.messageId].cb
-        that.outgoing[packet.messageId].cb = function (err, status) {
-          // Ensure that the original callback passed in to publish gets invoked
-          if (cb) {
-            cb(err, status)
+        cb = that.outgoing[packet.messageId] ? that.outgoing[packet.messageId].cb : null
+        that.outgoing[packet.messageId] = {
+          volatile: false,
+          cb: function (err, status) {
+            // Ensure that the original callback passed in to publish gets invoked
+            if (cb) {
+              cb(err, status)
+            }
+
+            storeDeliver()
           }
-
-          storeDeliver()
         }
         that._packetIdsDuringStoreProcessing[packet.messageId] = true
         that._sendPacket(packet)