quartz: don't call a NULL get_func() in gtk_clipboard_store()
quartz: don't call a NULL get_func() in gtk_clipboard_store()

Assume the clipboard is not set and bail out silently (bug #626499).

Distrotech/gtk,johne53/MB3Gtk-2,johne53/MB3Gtk-2,Distrotech/gtk,Distrotech/gtk,johne53/MB3Gtk-2,johne53/MB3Gtk-2,Distrotech/gtk,johne53/MB3Gtk-2,Distrotech/gtk