encoding/japanese: make EUCJP replace with FFFD on error

Updates golang/go#18898

Change-Id: Ic5fb77af67656889a387fa75a3e6efc9b9975817
Reviewed-on: https://go-review.googlesource.com/37316
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