encoding/japanese: make iso2022jp replace with FFFD on error

This CL also fixes a bug in the JIS 212 mode.

Updates golang/go#18898

Change-Id: Idb10e375bbc5d278db4e8550f26986a5d2bb6caa
Reviewed-on: https://go-review.googlesource.com/37595
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>
3 files changed