FP256BN Plus and math.MaxInt64

[ale-linux]

The following test

import (
	"math"
	"math/rand"
	"testing"

	"github.com/stretchr/testify/assert"
)

func TestMaxInt(t *testing.T) {
	r1 := int(rand.Int63())
	r2 := int(rand.Int63())

	i1 := FP256BN.NewBIGint(math.MaxInt64 - r1)
	i2 := FP256BN.NewBIGint(r1)
	i3 := FP256BN.NewBIGint(math.MaxInt64 - r2)
	i4 := FP256BN.NewBIGint(r2)
	i5 := FP256BN.NewBIGint(2)

	i6 := i1.Plus(i2).Plus(i3).Plus(i4).Plus(i5)

	zero := FP256BN.NewBIGint(0)

	assert.NotEqual(t, zero, i6)
}

fails whereas it looks like it should be succeeding. The test was conducted with go version go1.14.4 linux/amd64 against the curve

32. FP256BN

generated by config64.py (git revision 5387e6a895243dde0).