encoding/simplifiedchinese: make GB18030 replace with FFFD on error

Updates golang/go#18898

Change-Id: I0636d4ff317bbca015cf2a2a2ce9f452bdc601fd
Reviewed-on: https://go-review.googlesource.com/37322
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