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>