encoding/xml: Do not pass through invalid utf8 bytes

EscapeText now escapes 0xFFFD returned from DecodeRune as 0xFFFD, rather than passing through the original byte.
Fixes #5880.

R=golang-dev, r, bradfitz, adg
CC=golang-dev
https://golang.org/cl/11975043

src/pkg/encoding/xml/xml_test.go

@@ -11,6 +11,7 @@ import (
 	"reflect"
 	"strings"
 	"testing"
+	"unicode/utf8"
 )
 
 const testInput = `
@@ -714,3 +715,14 @@ func TestEscapeTextInvalidChar(t *testing.T) {
 		t.Errorf("have %v, want %v", text, expected)
 	}
 }
+
+func TestIssue5880(t *testing.T) {
+	type T []byte
+	data, err := Marshal(T{192, 168, 0, 1})
+	if err != nil {
+		t.Errorf("Marshal error: %v", err)
+	}
+	if !utf8.Valid(data) {
+		t.Errorf("Marshal generated invalid UTF-8: %x", data)
+	}
+}