encoding/korean: make EUCKR replace with FFFD on error

Updates golang/go#18898

Change-Id: I9868004acb11abbfee8492c9de8ba374f6dcb2ac
Reviewed-on: https://go-review.googlesource.com/37319
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