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
vips_foreign_load_init( VipsForeignLoad *load )
{
	load->disc = TRUE;
	load->access = VIPS_ACCESS_RANDOM;
}

vips_foreign_load_init( VipsForeignLoad *load )
{
	load->disc = TRUE;
	load->access = VIPS_ACCESS_RANDOM;
}
