playlist: playlist_RecursiveNodeSort() asserts the playlist lock

As any function that requires a valid playlist item as parameter, the
function can only make sense with the lock already held by the caller.

src/playlist/sort.c

@@ -193,6 +193,8 @@ static int recursiveNodeSort( playlist_t *p_playlist, playlist_item_t *p_node,
 int playlist_RecursiveNodeSort( playlist_t *p_playlist, playlist_item_t *p_node,
                                 int i_mode, int i_type )
 {
+    PL_ASSERT_LOCKED;
+
     /* Ask the playlist to reset as we are changing the order */
     pl_priv(p_playlist)->b_reset_currently_playing = true;