Invalid UTF-8 triggers an error when marshaling but, previously, not
when unmarshaling. This means that ASN.1 structures were not
round-tripping.

This change makes invalid UTF-8 in a string marked as UTF-8 to be an
error when Unmarshaling.

Fixes #11126.

Change-Id: Ic37be84d21dc5c03983525e244d955a8b1e1ff14
Reviewed-on: https://go-review.googlesource.com/11056
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>