internal/export/idna: always treat Punycode encoding pure ASCII as an error

UTS 46 revision 33 specifies that it is an error for a Punycode label to
encode an ASCII-only string, such as "xn--example-.com" encoding
"example.com".

Earlier versions of UTS 46 contained a specification bug which
permitted this.

Don't make the fix conditional on the Go toolchain version;
we should never accept these labels.

For golang/go#78760

Change-Id: I93beac3f44251e68a7faa3ea8b5963526a6a6964
Reviewed-on: https://go-review.googlesource.com/c/text/+/796420
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicholas Husin <nsh@golang.org>
Reviewed-by: Nicholas Husin <husin@google.com>

internal/export/idna/idna.go

diff --git a/internal/export/idna/idna.go b/internal/export/idna/idna.go
index 46e6e03..721a246 100644
--- a/internal/export/idna/idna.go
+++ b/internal/export/idna/idna.go
@@ -400,7 +400,11 @@
 				// Spec says keep the old label.
 				continue
 			}
-			if unicode16 && err == nil && len(u) > 0 && isASCII(u) {
+			if err == nil && len(u) > 0 && isASCII(u) {
+				// UTS 43 pre-revision 33 doesn't classify a xn-- label
+				// which contains only ASCII characters as an error,
+				// but that's a specification bug and a security issue.
+				// Always return an error in this case.
 				err = punyError(enc)
 			}
 			isBidi = isBidi || bidirule.DirectionString(u) != bidi.LeftToRight