[kimchi][prover] less error-prone way to get the zk rows

dlog/plonk-15-wires/src/prover.rs

@@ -140,7 +140,7 @@ where
                 padded_witness[..w.len()].copy_from_slice(w);
 
                 // add the zero-knowledge rows
-                for w_ in &mut padded_witness[length_without_zk_rows..] {
+                for w_ in padded_witness.iter_mut().rev().take(ZK_ROWS as usize) {
                     *w_ = Fr::<G>::rand(rng);
                 }