When loading a nonexisting CSS file using
gtk_css_provider_load_from_file() or gtk_css_provider_load_from_path()
we would emit the error using a NULL scanner. Don't do that, because
we'll have a NULL section in that case and error handlers don't like
that.
Testcase attached.
https://bugzilla.redhat.com/show_bug.cgi?id=1277959