big: prevent errors in Exp in the face of aliasing

R=gri
CC=golang-dev, golang-dev
https://golang.org/cl/1244044

src/pkg/big/int.go

@@ -434,8 +434,9 @@ func (z *Int) BitLen() int {
 // See Knuth, volume 2, section 4.6.3.
 func (z *Int) Exp(x, y, m *Int) *Int {
 	if y.neg || len(y.abs) == 0 {
+		neg := x.neg
 		z.SetInt64(1)
-		z.neg = x.neg
+		z.neg = neg
 		return z
 	}
 

src/pkg/big/int_test.go

@@ -602,7 +602,7 @@ func TestExp(t *testing.T) {
 			continue
 		}
 
-		z := new(Int).Exp(x, y, m)
+		z := y.Exp(x, y, m)
 		if !isNormalized(z) {
 			t.Errorf("#%d: %v is not normalized", i, *z)
 		}

src/pkg/big/nat.go

@@ -920,6 +920,11 @@ func (z nat) random(rand *rand.Rand, limit nat, n int) nat {
 // If m != nil, expNN calculates x**y mod m. Otherwise it calculates x**y. It
 // reuses the storage of z if possible.
 func (z nat) expNN(x, y, m nat) nat {
+	if alias(z, x) || alias(z, y) {
+		// We cannot allow in place modification of x or y.
+		z = nil
+	}
+
 	if len(y) == 0 {
 		z = z.make(1)
 		z[0] = 1