internal/export/idna: reject non-ASCII in xn-- labels

[cristiangirlea]

decode copied the basic code points of a Punycode string without
checking that they are ASCII, so a label such as "xn--が-" decoded to
"が" and converted to the valid A-label "xn--v8j". RFC 3492 section
6.2 makes such input invalid, and UTS #46 processing step 4 records
an error for an xn-- label that contains non-ASCII.

Reject non-ASCII input in decode.

The new tests fail before the fix. internal/export/idna passes on
linux/amd64, including the UTS #46 conformance test with -long, and
on linux/386 and with -race. With the change copied to x/net/idna,
the example from the issue now returns an error.

Fixes golang/go#81116

[gopherbot]

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