safer stop-early logic in tilecache
safer stop-early logic in tilecache

Don't actually stop the tilecache loop early, since that could deadlock,
instead just stop fetching from the pipeline.

see https://github.com/libvips/libvips/pull/2424

lovell/libvips,lovell/libvips,jcupitt/libvips,jcupitt/libvips,lovell/libvips,jcupitt/libvips,jcupitt/libvips,lovell/libvips