In gimp_image_merge_layers(), explicitly fetch the graph of the top
layer's parent layer (if exists), to make sure that the top layer's
graph has a parent node.  We already fetch the image graph, which
takes care of top-level layers, however, if the top layer is a
child of an invisible layer group, as is the case in the wavelet-
decompose plug-in, this is not generally enough to guarantee that
the group's graph is constructed.