Fix invalid call to st_widget_get_theme_node() in WindowOverlay

WindowOverlay has two actors, both with custom style properties, which
share a common _onStyleChanged() handler. This is not a problem when
entering the overview, because the actors' parent (the workspaces group)
is hidden while the actors are added. However, when windows are added to
the workspace while in the overview (e.g. when opening a new window or
dragging a window from one workspace to another), adding the first actor
to the workspaces group triggers a style-changed signal - the handler
then calls st_widget_get_theme_node() on both actors, which triggers a
warning as the second actor has not been parented yet.

https://bugzilla.gnome.org/show_bug.cgi?id=610279