encoding/simplifiedchinese: make HZGB2312 replace with FFFD on error

Not sure if the right number of replacement characters is
returned in all instances, but seems reasonable.

Updates golang/go#18898

Change-Id: Ibf6efdb079191aa6db4eb05b41b7dae593947bd0
Reviewed-on: https://go-review.googlesource.com/37324
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