fixed memory leak in ringbuffer after destroying client

mqtt.c

@@ -487,6 +487,7 @@ void mqtt_destroy(mqtt_client *client)
 
     free(client->mqtt_state.in_buffer);
     free(client->mqtt_state.out_buffer);
+    free(client->send_rb.p_o);
     free(client);
 
     mqtt_info("Client destroyed");