The result of converting the IP address of the broker wasn't checked. As a result, the pointer was left uninitialised and the IPv6 address used for connecting was some random data. The function now returns an error. Before connect_to_broker is called, mqtt_register is executed, which memsets conn to 0, making its state 0 (MQTT_CONN_STATE_ERROR). In order to recover from this error state, the extra check was added in the MQTT_CLIENT_STATE_NEWCONFIG state.
This was discovered using [CodeSonar](https://www.grammatech.com/products/codesonar)
The current version of the CC26xx web demo publishes over MQTT the default parent's IPv6 address and the last observed RSSI of this link. This is collected by active probing (periodic ping).
This commit brings the probing functionality to the example's main code module. The MQTT client keeps publishing as previously, but we now also export the same information through CoAP resources. Configuration is still possible through the example's web server.
