Fixed two memory leaks when creating folderitems.

Fixed two memory leaks when creating folderitems.

The Folderitem's "node" member is set to a new empty
GNode in folder_item_new(), but is later set to
another newly allocated GNode.

This commit removes the extra allocation in one case
(folder_new()), and gets rid of the original GNode
in another case, where avoiding the second allocation
is impossible without completely rewriting the recursive
folderitem appending in folder_get_from_xml().