fix invalidate tagging
fix invalidate tagging

The new tagging system was unreffing the wrong operation.

See https://github.com/libvips/libvips/issues/1494#issuecomment-565200939

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