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

diff --git a/ssh/kex.go b/ssh/kex.go
index 1607200..6c3c648 100644
--- a/ssh/kex.go
+++ b/ssh/kex.go
@@ -212,7 +212,7 @@
 		HostKey:   hostKeyBytes,
 		Signature: sig,
 		Hash:      crypto.SHA1,
-	}, nil
+	}, err
 }
 
 // ecdh performs Elliptic Curve Diffie-Hellman key exchange as