Updates based on peer review. Added checks for null and return code.

Author: dgarske

examples/mqttclient/mqttclient.c

@@ -153,7 +153,11 @@ int mqttclient_test(MQTTCtx *mqttCtx)
 
         #ifdef WOLFMQTT_DISCONNECT_CB
             /* setup disconnect callback */
-            MqttClient_SetDisconnectCallback(&mqttCtx->client, mqtt_disconnect_cb, NULL);
+            rc = MqttClient_SetDisconnectCallback(&mqttCtx->client,
+                mqtt_disconnect_cb, NULL);
+            if (rc != MQTT_CODE_SUCCESS) {
+                goto exit;
+            }
         #endif
 
             FALL_THROUGH;

src/mqtt_packet.c

@@ -703,7 +703,8 @@ static int MqttPacket_HandleNetError(MqttClient *client, int rc)
         rc != MQTT_CODE_STDIN_WAKE)
     {
         /* don't use return code for now - future use */
-        client->disconnect_cb(client, rc, client->disconnect_ctx);
+        if (client->disconnect_cb)
+            client->disconnect_cb(client, rc, client->disconnect_ctx);
     }
 #endif
     return rc;