mqttbutton: Fix MQTT disconnect handler

When the MQTT disconnect handler is called, it would crash the
microcontroller because it was attempting to call `close` on a nil value
`client`. This was supposed to have been `mqtt_client`.

Immediately calling the WiFi timer callback is also a problem, as it
effectively puts the microcontroller into a busy loop if the WiFi is
already connected but the MQTT broker is unavailable. Adding a 1 second
delay here should prevent crashing in this case.

mqttbutton.lua

@@ -43,8 +43,8 @@ end
 
 function on_mqtt_disconnected(client)
     print("MQTT Disconnected")
-    client:close()
+    mqtt_client:close()
-    on_wifi_timer()
+    tmr.alarm(WIFI_TIMER_ID, 1000, 0, on_wifi_timer)
 end
 
 function on_wifi_connected()