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_save_init( VipsForeignSave *save )
{
	save->background = vips_array_double_newv( 1, 0.0 );
}

vips_foreign_save_init( VipsForeignSave *save )
{
	save->background = vips_array_double_newv( 1, 0.0 );
}
