fix int overflow in vips_region_copy
fix int overflow in vips_region_copy

this could cause crashes with very wide images, see:

https://github.com/libvips/libvips/issues/1989

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