Handle duplicate PUBREL

Fixes #138

If the connection to a broker is lost mid qos 2 flow, there is a window where we have processed the PUBREL, released the message and deleted it from our store, but not sent the PUBCOMP. When the connection is re-established, and the PUBREL is resent by the broker, we assume the message still exists - and hit the error reported.

The fix is to check the message is valid before trying to process it. We send the PUBCOMP to complete the flow regardless.

nodes/core/io/lib/mqtt.js

@@ -130,8 +130,10 @@ MQTTClient.prototype.connect = function(options) {
          client.on('pubrel',function(packet) {
                self.lastInbound = (new Date()).getTime()
                var p = self.inboundMessages[packet.messageId];
+               if (p) {
                    self.emit('message',p.topic,p.payload,p.qos,p.retain);
                    delete self.inboundMessages[packet.messageId];
+               }
                self.lastOutbound = (new Date()).getTime()
                self.client.pubcomp(packet);
          });