x11: Check return of gdk_x11_get_xatom_name_for_display
When given an invalid atom, gdk_x11_get_xatom_name_for_display can return NULL and trigger a seg in gdk_x11_clipboard_formats_from_atoms. Check for NULL.
Why I'm seeing a bad atom there is probably a separate question. (Although my nice repeatable test case has just stopped repeating) https://bugzilla.redhat.com/show_bug.cgi?id=2037786