if mqtt disconnected save memory

src/mqtt.cpp

@@ -74,6 +74,10 @@ void Mqtt::subscribe(const uint8_t device_type, const std::string & topic, mqtt_
     // add to MQTT queue as a subscribe operation
     auto message = queue_subscribe_message(topic);
 
+    if (message == nullptr) {
+        return;
+    }
+
     // register in our libary with the callback function.
     // We store both the original topic and the fully-qualified one
     mqtt_subfunctions_.emplace_back(device_type, std::move(topic), std::move(message->topic), std::move(cb));
@@ -556,7 +560,7 @@ std::shared_ptr<const MqttMessage> Mqtt::queue_message(const uint8_t operation,
 
 // add MQTT message to queue, payload is a string
 std::shared_ptr<const MqttMessage> Mqtt::queue_publish_message(const std::string & topic, const std::string & payload, bool retain) {
-    if (!enabled()) {
+    if (!enabled() || !connected()) {
         return nullptr;
     };
     return queue_message(Operation::PUBLISH, topic, payload, retain);