ssh/agent: Fix error returned from agent responses that are too big.

Make sure a meaningful error is returned when the SSH agent client receives
a response that is too big.

Change-Id: Ied68b2c559a61dede2f3112c4c8143babd385f48
GitHub-Last-Rev: ad65e1b423bb00f2501dfde84040469aef4ee2df
GitHub-Pull-Request: golang/crypto#61
Reviewed-on: https://go-review.googlesource.com/c/143058
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

ssh/agent/client.go

@@ -348,7 +348,7 @@ func (c *client) callRaw(req []byte) (reply []byte, err error) {
 	}
 	respSize := binary.BigEndian.Uint32(respSizeBuf[:])
 	if respSize > maxAgentResponseBytes {
-		return nil, clientErr(err)
+		return nil, clientErr(errors.New("response too large"))
 	}
 
 	buf := make([]byte, respSize)