idna: update from x/text

[cristiangirlea]

Changes produced by running the copyExported("golang.org/x/net/idna")
step of go generate in golang.org/x/text at 6dcc5b6.

This brings in CL 818802, which rejects xn-- labels that decode to an
empty label, as UTS #46 processing step 4 requires. Before, a name
such as "example.com.xn--" converted to "example.com." without an
error. It also brings in CL 818800 and CL 818801.

idna, http/httpguts, http/httpproxy, http2 and publicsuffix pass on
linux/amd64, also with -race, and idna on linux/386.

Fixes golang/go#81549

[gopherbot]

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