mqtt: Break out of receive loop on disconnect

Apparently, the `AsyncReceiver` stream produces nested `Option` objects.
The outer is `None` if "the stream is exhausted," which is somehow
different than the connection being closed; the inner `Option` is `None`
in that case.

We were originally ignoring the inner `None`, but just causes the
async task to go into a busy loop when connection is closed.  We need to
break out of the loop there instead.

src/mqtt.rs

@@ -103,7 +103,7 @@ impl<'a> MqttClient<'a> {
             client.disconnect(None);
         });
         while let Some(msg) = self.stream.next().await {
-            let Some(msg) = msg else {continue};
+            let Some(msg) = msg else {break};
             trace!("Received message: {:?}", msg);
             for m in self.topics.matches(msg.topic()) {
                 match m.1 {