encoding/xml: fix panic in Marshal

Fixes #6341.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/13512048
diff --git a/src/pkg/encoding/xml/marshal_test.go b/src/pkg/encoding/xml/marshal_test.go
index 8d9239e..6cd894e 100644
--- a/src/pkg/encoding/xml/marshal_test.go
+++ b/src/pkg/encoding/xml/marshal_test.go
@@ -904,6 +904,10 @@
 	X string `xml:"X>Y,attr"`
 }
 
+type BadAttr struct {
+	Name []string `xml:"name,attr"`
+}
+
 var marshalErrorTests = []struct {
 	Value interface{}
 	Err   string
@@ -936,6 +940,10 @@
 		Value: &AttrParent{},
 		Err:   `xml: X>Y chain not valid with attr flag`,
 	},
+	{
+		Value: BadAttr{[]string{"X", "Y"}},
+		Err:   `xml: unsupported type: []string`,
+	},
 }
 
 var marshalIndentTests = []struct {