x/net/idna: fix checking hyphen at 3rd and 4th character

[hnakamur]

Check hyphen restrictions according to Section 4.2.3.1 of
RFC 5891.

4.2.3.1. Hyphen Restrictions
The Unicode string MUST NOT contain "--" (two consecutive
hyphens) in the third and fourth character positions and
MUST NOT start or end with a "-" (hyphen).

https://www.rfc-editor.org/info/rfc5891/#section-4.2.3.1

Fixes golang/go#81026

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

This PR (HEAD: 84cb650) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/net/+/819700.

Important tips:

• Don't comment on this PR. All discussion takes place in Gerrit.
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[gopherbot]

Message from Hiroaki Nakamura:

Patch Set 1:

(2 comments)

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