ssh: close net.Conn on all NewServerConn errors

This PR ensures that the net.Conn passed to ssh.NewServerConn is closed
on all error return paths, not just after a failed handshake. This matches
the behavior of ssh.NewClientConn.

Change-Id: Id8a51d10ae8d575cbbe26f2ef6b37de7cca840ec
GitHub-Last-Rev: 81bb2e58a881a9a85935740bda06b034b32a8ce3
GitHub-Pull-Request: golang/crypto#279
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/549095
Run-TryBot: Nicola Murino <nicola.murino@gmail.com>
Auto-Submit: Nicola Murino <nicola.murino@gmail.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Nicola Murino <nicola.murino@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

ssh/server.go

@@ -213,6 +213,7 @@ func NewServerConn(c net.Conn, config *ServerConfig) (*ServerConn, <-chan NewCha
 	} else {
 		for _, algo := range fullConf.PublicKeyAuthAlgorithms {
 			if !contains(supportedPubKeyAuthAlgos, algo) {
+				c.Close()
 				return nil, nil, nil, fmt.Errorf("ssh: unsupported public key authentication algorithm %s", algo)
 			}
 		}
@@ -220,6 +221,7 @@ func NewServerConn(c net.Conn, config *ServerConfig) (*ServerConn, <-chan NewCha
 	// Check if the config contains any unsupported key exchanges
 	for _, kex := range fullConf.KeyExchanges {
 		if _, ok := serverForbiddenKexAlgos[kex]; ok {
+			c.Close()
 			return nil, nil, nil, fmt.Errorf("ssh: unsupported key exchange %s for server", kex)
 		}
 	}