fix a crash with delayed load

If a delayed load failed, it could leave the pipeline only half-set up.
Sebsequent threads could then segv.

Set a load-has-failed flag and test before generate.

See https://github.com/jcupitt/libvips/issues/893
file_add_class( VipsForeignClass *class, GSList **files )
{
	/* Append so we don't reverse the list of files. Sort will not reorder
	 * items of equal priority. 
	 */
	*files = g_slist_append( *files, class );

	return( NULL );
}

file_add_class( VipsForeignClass *class, GSList **files )
{
	/* Append so we don't reverse the list of files. Sort will not reorder
	 * items of equal priority. 
	 */
	*files = g_slist_append( *files, class );

	return( NULL );
}
