The msg[] array is defined as 50 bytes long, but then the snprintf is
protected at 75 bytes long, which is incorrectly 25 bytes longer than
the underlying array.
To correct this, define a new macro MSG_BUFFER_SIZE and use it in both
places:
* defining msg[]
* writing to msg[] with snprintf()
Many users load the sketch having only changed WiFi SSID and password. When multiple users attempt to connect to the same broker using same client ID, they get rejected. Currently the chances of connecting to broker.mqtt-dashboard.com using "ESP8266Client" ID are fairly slim. This change adds a random number to the client ID, increasing chances of connection for new users of this library.
