Zerocheck didn't support constant expr.

Author: dreamATD

subprotocols/src/zerocheck.rs

@@ -270,8 +270,11 @@ where
                         let len = poly.len() + 1;
                         // last_sigma = (1 - point[round]) * eval_at_0 + point[round] * eval_at_1
                         // eval_at_0 = (last_sigma - point[round] * eval_at_1) * inv(1 - point[round])
-                        let eval_at_0 =
-                            (last_sigma - point[round] * poly[0]) * inv_of_one_minus_point[round];
+                        let eval_at_0 = if !poly.is_empty() {
+                            (last_sigma - point[round] * poly[0]) * inv_of_one_minus_point[round]
+                        } else {
+                            last_sigma
+                        };
 
                         // Evaluations on degree, degree - 1, ..., 1, 0.
                         let evals_iter_rev = chain![poly.into_iter().rev(), iter::once(eval_at_0)];