ssh: fix error message on unsupported cipher

[porridgewithraisins]

Until now, when ssh keys using one of these[1] ciphers were passed, we were
giving a parse error "ssh: parse error in message type 0".

With this fix, we parse it successfully and return the correct error message.

[1] aes{128,256}-gcm@openssh.com and chacha20-poly1305@openssh.com

Fixes golang/go#52135

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[porridgewithraisins]

Have signed the CLA, please review.

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Patch Set 1:

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Patch Set 1:

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Patch Set 3: Commit-Queue+1

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