encoding/traditionalchinese: make Big5 replace with FFFD on error

Updates golang/go#18898

Change-Id: If234aa5fdc35daf5ab02f49400462aa0c1ffa5ea
Reviewed-on: https://go-review.googlesource.com/37325
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