fix: Uses caps allocation for data buffer instead of item struct

Once introduced the memory destination for outbox was incorrectly
allocating the outbox data structuture instead of data buffer to the
selected memory.

lib/mqtt_outbox.c

@@ -41,15 +41,15 @@ outbox_handle_t outbox_init(void)
 
 outbox_item_handle_t outbox_enqueue(outbox_handle_t outbox, outbox_message_handle_t message, outbox_tick_t tick)
 {
-    outbox_item_handle_t item = heap_caps_calloc(1, sizeof(outbox_item_t), MQTT_OUTBOX_MEMORY);
+    outbox_item_handle_t item = calloc(1, sizeof(outbox_item_t));
     ESP_MEM_CHECK(TAG, item, return NULL);
     item->msg_id = message->msg_id;
     item->msg_type = message->msg_type;
     item->msg_qos = message->msg_qos;
     item->tick = tick;
     item->len =  message->len + message->remaining_len;
     item->pending = QUEUED;
-    item->buffer = malloc(message->len + message->remaining_len);
+    item->buffer = heap_caps_malloc(message->len + message->remaining_len, MQTT_OUTBOX_MEMORY);
     ESP_MEM_CHECK(TAG, item->buffer, {
         free(item);
         return NULL;