[queue] If the playing item is not in the queue anymore, the next item

should be the first item in the queue

This solves the problem that after clearing the queue and adding new
items while playing, aborts playback when skipped to the next item.

src/queue.c

@@ -460,8 +460,8 @@ queue_next(struct queue *queue, unsigned int item_id, char shuffle, enum repeat_
   item = queueitem_get_byitemid(queue, item_id);
 
   if (!item)
-    // Item not found
-    return NULL;
+    // Item not found, start playing from the start of the queue
+    item = queue->head;
 
   if (r_mode == REPEAT_SONG && item != queue->head)
     return item;