fix #1410.

fix #1410.
You can not change a playlist_item_t to be a node, when you previously registered events on it, using input_item_t sub items adding event, because the pointer will be destroyed.
If you want to use nodes anyway, you have to explicitely create them.
Note: we were lucky that xspf did work so far, because we were EXCLUSIVELY using invalid pointers, except for the 1st item being added to each playlist_item_t (it did happen before the conversion obviously).