Ensure multiexp never uses a zero-weight in its batch verifier

crypto/multiexp/src/batch.rs

@@ -22,7 +22,12 @@ impl<Id: Copy, G: Group> BatchVerifier<Id, G> where <G as Group>::Scalar: PrimeF
     let u = if self.0.len() == 0 {
       G::Scalar::one()
     } else {
-      G::Scalar::random(rng)
+      let mut weight = G::Scalar::random(&mut *rng);
+      // Ensure it's non-zero, as a zero scalar would cause this item to pass no matter what
+      while weight.is_zero().into() {
+        weight = G::Scalar::random(&mut *rng);
+      }
+      weight
     };
     self.0.push((id, pairs.into_iter().map(|(scalar, point)| (scalar * u, point)).collect()));
   }