crypto/cloudflare/bn256: fix in-place addition and unmarshalling (#23419)

crypto/bn256/cloudflare/curve.go

@@ -171,15 +171,15 @@ func (c *curvePoint) Double(a *curvePoint) {
 	gfpAdd(t, d, d)
 	gfpSub(&c.x, f, t)
 
+	gfpMul(&c.z, &a.y, &a.z)
+	gfpAdd(&c.z, &c.z, &c.z)
+
 	gfpAdd(t, C, C)
 	gfpAdd(t2, t, t)
 	gfpAdd(t, t2, t2)
 	gfpSub(&c.y, d, &c.x)
 	gfpMul(t2, e, &c.y)
 	gfpSub(&c.y, t2, t)
-
-	gfpMul(t, &a.y, &a.z)
-	gfpAdd(&c.z, t, t)
 }
 
 func (c *curvePoint) Mul(a *curvePoint, scalar *big.Int) {