big: fix nat.scan bug

Scanning "0" with detected base did not actually set the nat to 0.

R=gri
CC=golang-dev
https://golang.org/cl/4923050

src/pkg/big/int_test.go

@@ -301,6 +301,9 @@ func TestGetString(t *testing.T) {
 func TestSetString(t *testing.T) {
 	tmp := new(Int)
 	for i, test := range stringTests {
+		// initialize to a non-zero value so that issues with parsing
+		// 0 are detected
+		tmp.SetInt64(1234567890)
 		n1, ok1 := new(Int).SetString(test.in, test.base)
 		n2, ok2 := tmp.SetString(test.in, test.base)
 		expected := NewInt(test.val)

src/pkg/big/nat.go

@@ -646,7 +646,7 @@ func (z nat) scan(r io.RuneScanner, base int) (nat, int, os.Error) {
 					}
 				}
 			case os.EOF:
-				return z, 10, nil
+				return z.make(0), 10, nil
 			default:
 				return z, 10, err
 			}