bookmarkfile: Don't move an item if the uri has not changed

This was causing a crash, because we were first removing an item, freeing
both the instance itself and the key, and then trying to reuse those.

So, in this case, instead of reassigning an item, we can just return TRUE
as we have already the item at the right place, while it's not needed to
update the modified timestamp, since no modification happened in reality.

Fixes #1588

glib/gbookmarkfile.c

@@ -3584,6 +3584,9 @@ g_bookmark_file_move_item (GBookmarkFile  *bookmark,
 
   if (new_uri && new_uri[0] != '\0')
     {
+      if (g_strcmp0 (old_uri, new_uri) == 0)
+        return TRUE;
+
       if (g_bookmark_file_has_item (bookmark, new_uri))
         {
           if (!g_bookmark_file_remove_item (bookmark, new_uri, error))