MediaListEndReached event should not be sent only when a new sub item tree is
added, otherwise it won't be sent if media doesn't have any sub items.

To fix this issue, send the MediaListEndReached event when preparse function
terminates (only if the media_list was created either by user or by sub items
callbacks).
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>