crypto/bn256: fix MulScalar (#30974)

The `a` parameter should be used in the `MulScalar` function. The
upstream cloudflare and google repos have already merged fixes.

Reference:
*
8d7daa0c54
* https://github.com/cloudflare/bn256/pull/33

crypto/bn256/cloudflare/gfp12.go

@@ -105,8 +105,8 @@ func (e *gfP12) Mul(a, b *gfP12) *gfP12 {
 }
 
 func (e *gfP12) MulScalar(a *gfP12, b *gfP6) *gfP12 {
-	e.x.Mul(&e.x, b)
-	e.y.Mul(&e.y, b)
+	e.x.Mul(&a.x, b)
+	e.y.Mul(&a.y, b)
 	return e
 }
 

crypto/bn256/google/gfp12.go

@@ -125,8 +125,8 @@ func (e *gfP12) Mul(a, b *gfP12, pool *bnPool) *gfP12 {
 }
 
 func (e *gfP12) MulScalar(a *gfP12, b *gfP6, pool *bnPool) *gfP12 {
-	e.x.Mul(e.x, b, pool)
-	e.y.Mul(e.y, b, pool)
+	e.x.Mul(a.x, b, pool)
+	e.y.Mul(a.y, b, pool)
 	return e
 }