Use point.is_zero() for allocating infinity in SW curves (#178)

Author: Pratyush

src/groups/curves/short_weierstrass/bls12/mod.rs

@@ -67,7 +67,7 @@ impl<P: Bls12Config> AllocVar<G1Prepared<P>, P::Fp> for G1PreparedVar<P> {
         let y = FpVar::new_variable(ark_relations::ns!(cs, "y"), || g1_prep.map(|g| g.y), mode)?;
         let infinity = Boolean::new_variable(
             ark_relations::ns!(cs, "inf"),
-            || g1_prep.map(|g| g.infinity),
+            || g1_prep.map(|g| g.is_zero()),
             mode,
         )?;
         let g = AffineVar::new(x, y, infinity);

src/groups/curves/short_weierstrass/mod.rs

@@ -165,7 +165,7 @@ where
             let point = self.value()?.into_affine();
             let x = F::new_constant(ConstraintSystemRef::None, point.x)?;
             let y = F::new_constant(ConstraintSystemRef::None, point.y)?;
-            let infinity = Boolean::constant(point.infinity);
+            let infinity = Boolean::constant(point.is_zero());
             Ok(AffineVar::new(x, y, infinity))
         } else {
             let cs = self.cs();