heifload: simplify is_a check of first 4 bytes
heifload: simplify is_a check of first 4 bytes

Allow multiples of 4, up to 32, as chunk length

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