ssh/agent: return an error for unexpected message types

Previously, receiving an unexpected message type in response to a key
listing or a signing request could cause a panic due to a failed type
assertion.

This change adds a default case to the type switch in order to detect
and explicitly handle unknown or invalid message types, returning a
descriptive error instead of crashing.

Fixes golang/go#75178

Change-Id: Icbc3432adc79fe3c56b1ff23c6724d7a6f710f3a
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/700295
Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Jakub Ciolek <jakub@ciolek.dev>

ssh/agent/client.go

@@ -430,8 +430,9 @@ func (c *client) List() ([]*Key, error) {
 		return keys, nil
 	case *failureAgentMsg:
 		return nil, errors.New("agent: failed to list keys")
+	default:
+		return nil, fmt.Errorf("agent: failed to list keys, unexpected message type %T", msg)
 	}
-	panic("unreachable")
 }
 
 // Sign has the agent sign the data using a protocol 2 key as defined
@@ -462,8 +463,9 @@ func (c *client) SignWithFlags(key ssh.PublicKey, data []byte, flags SignatureFl
 		return &sig, nil
 	case *failureAgentMsg:
 		return nil, errors.New("agent: failed to sign challenge")
+	default:
+		return nil, fmt.Errorf("agent: failed to sign challenge, unexpected message type %T", msg)
 	}
-	panic("unreachable")
 }
 
 // unmarshal parses an agent message in packet, returning the parsed