openpgp/clearsign: fix minor unreachable code caused by t.Fatalf

[Abirdcfly]

[gopherbot]

This PR (HEAD: c606bc6d83e6e9720afe40dadf112389b331414d) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/crypto/+/418855 to see it.

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