macosx: fix crash when re-ordering playlist items

modules/gui/macosx/playlist.m

@@ -1478,6 +1478,9 @@
 
     /* Drag & Drop inside the playlist */
     if ([[o_pasteboard types] containsObject: @"VLCPlaylistItemPboardType"]) {
+        if (index == -1) // this is no valid target, sanitize to top of table
+            index = 0;
+
         int i_row = 0;
         playlist_item_t *p_new_parent, *p_item = NULL;
         NSArray *o_all_items = [o_nodes_array arrayByAddingObjectsFromArray: o_items_array];