go/scanner: clear all fields after Scanner reuse

go/scanner: clear all fields after Scanner reuse

We were missing s.nlPos = token.NoPos in Init, but while we are here
let's make it less likely to hit this it in future.

Change-Id: Ief4c0ba2cf97bc556d901eabc8e172406a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/738680
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>