ssh: fix dropped error

This fixes an error variable that was being dropped
prior to the return of dhGroup.Server().

Change-Id: I8ddef1832cd7a62a69b61bb1c98bc1ceb561f985
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/205420
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

ssh/kex.go

@@ -212,7 +212,7 @@ func (group *dhGroup) Server(c packetConn, randSource io.Reader, magics *handsha
 		HostKey:   hostKeyBytes,
 		Signature: sig,
 		Hash:      crypto.SHA1,
-	}, nil
+	}, err
 }
 
 // ecdh performs Elliptic Curve Diffie-Hellman key exchange as