math/big: fix destination leak into result value

This code would panic:
z := big.NewInt(1)
z.SetBit(big.NewInt(0), 2, 1)
if z.Cmp(big.NewInt(1<<2)) != 0 {
        panic("fail")
}

R=rsc, gri
CC=golang-dev
https://golang.org/cl/5437081

src/pkg/math/big/int_test.go

@@ -1242,10 +1242,14 @@ func TestBitSet(t *testing.T) {
 		x.SetString(test.x, 0)
 		b := x.Bit(test.i)
 		if b != test.b {
-
-			t.Errorf("#%d want %v got %v", i, test.b, b)
+			t.Errorf("#%d got %v want %v", i, b, test.b)
 		}
 	}
+	z := NewInt(1)
+	z.SetBit(NewInt(0), 2, 1)
+	if z.Cmp(NewInt(4)) != 0 {
+		t.Errorf("destination leaked into result; got %s want 4", z)
+	}
 }
 
 func BenchmarkBitset(b *testing.B) {

src/pkg/math/big/nat.go

@@ -1065,9 +1065,11 @@ func (z nat) setBit(x nat, i uint, b uint) nat {
 		return z.norm()
 	case 1:
 		if j >= n {
-			n = j + 1
-		}
+			z = z.make(j + 1)
+			z[n:].clear()
+		} else {
 			z = z.make(n)
+		}
 		copy(z, x)
 		z[j] |= m
 		// no need to normalize