encoding/japanese: make ShiftJIS replace with FFFD on error

Updates golang/go#18898

Change-Id: I049e5ba1fca9529eeacc3aa58f7e5c2d17f22ecd
Reviewed-on: https://go-review.googlesource.com/37317
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>
2 files changed