fix map of custom seekable streams
fix map of custom seekable streams

We failed for seekable custom streams.

See https://github.com/libvips/libvips/issues/1494#issuecomment-567190830

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