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
diff --git a/src/pkg/big/nat.go b/src/pkg/big/nat.go
index be3aff2..33d6bb1 100755
--- a/src/pkg/big/nat.go
+++ b/src/pkg/big/nat.go
@@ -646,7 +646,7 @@
 					}
 				}
 			case os.EOF:
-				return z, 10, nil
+				return z.make(0), 10, nil
 			default:
 				return z, 10, err
 			}