Make SSH_FXP_STATUS error message optional (#109)

Some SSH implementations do not include an error message in the
SSH_FXP_STATUS response causing an "index out of range" panic.

client.go

@@ -1165,7 +1165,10 @@ func unmarshalStatus(id uint32, data []byte) error {
 		return &unexpectedIDErr{id, sid}
 	}
 	code, data := unmarshalUint32(data)
-	msg, data := unmarshalString(data)
+	msg, data, err := unmarshalStringSafe(data)
+	if err != nil {
+		return err
+	}
 	lang, _, _ := unmarshalStringSafe(data)
 	return &StatusError{
 		Code: code,

client_test.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"io"
 	"os"
+	"reflect"
 	"testing"
 
 	"github.com/kr/fs"
@@ -87,13 +88,58 @@ func TestFlags(t *testing.T) {
 	}
 }
 
-func TestMissingLangTag(t *testing.T) {
-	defer func() {
-		if r := recover(); r != nil {
-			t.Fail()
+func TestUnmarshalStatus(t *testing.T) {
+	requestID := uint32(1)
+
+	id := marshalUint32([]byte{}, requestID)
+	idCode := marshalUint32(id, ssh_FX_FAILURE)
+	idCodeMsg := marshalString(idCode, "err msg")
+	idCodeMsgLang := marshalString(idCodeMsg, "lang tag")
+
+	var tests = []struct {
+		desc   string
+		reqID  uint32
+		status []byte
+		want   error
+	}{
+		{
+			desc:   "well-formed status",
+			reqID:  1,
+			status: idCodeMsgLang,
+			want: &StatusError{
+				Code: ssh_FX_FAILURE,
+				msg:  "err msg",
+				lang: "lang tag",
+			},
+		},
+		{
+			desc:   "missing error message and language tag",
+			reqID:  1,
+			status: idCode,
+			want:   errShortPacket,
+		},
+		{
+			desc:   "missing language tag",
+			reqID:  1,
+			status: idCodeMsg,
+			want: &StatusError{
+				Code: ssh_FX_FAILURE,
+				msg:  "err msg",
+			},
+		},
+		{
+			desc:   "request identifier mismatch",
+			reqID:  2,
+			status: idCodeMsgLang,
+			want:   &unexpectedIDErr{2, requestID},
+		},
+	}
+
+	for _, tt := range tests {
+		got := unmarshalStatus(tt.reqID, tt.status)
+		if !reflect.DeepEqual(got, tt.want) {
+			t.Errorf("unmarshalStatus(%v, %v), test %q\n- want: %#v\n-  got: %#v",
+				requestID, tt.status, tt.desc, tt.want, got)
 		}
-	}()
-	buf := marshalUint32([]byte{}, 0)
-	buf = marshalStatus(buf, StatusError{})
-	_ = unmarshalStatus(0, buf[:len(buf)-4])
+	}
 }