Possible nullptr dereference in esp_event_handler_register_with() (IDFGH-2224)

[AxelLin]

esp-idf 4.0 beta2:
In esp_event/esp_event.c, esp_event_handler_register_with function:

The if (!loop_node) test needs to be done before SLIST_INIT.

` if (!last_loop_node ||
(last_loop_node && !SLIST_EMPTY(&(last_loop_node->base_nodes)) && is_loop_level_handler)) {
loop_node = (esp_event_loop_node_t*) calloc(1, sizeof(*loop_node));

    SLIST_INIT(&(loop_node->handlers));
    SLIST_INIT(&(loop_node->base_nodes));

    if (!loop_node) {
        ESP_LOGE(TAG, "alloc for new loop node failed");
        err = ESP_ERR_NO_MEM;
        goto on_err;
    }

`