encoding/xml: don't crash on invalid XMLName tag

Fixes #20953

Change-Id: Ia30a6e0e335c1f738e1359500e09057b5981f1c7
Reviewed-on: https://go-review.googlesource.com/82397
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/encoding/xml/marshal_test.go

@@ -2441,3 +2441,22 @@ func TestIssue16158(t *testing.T) {
 		t.Errorf("Unmarshal: expected error, got nil")
 	}
 }
+
+// Issue 20953. Crash on invalid XMLName attribute.
+
+type InvalidXMLName struct {
+	XMLName Name `xml:"error"`
+	Type    struct {
+		XMLName Name `xml:"type,attr"`
+	}
+}
+
+func TestInvalidXMLName(t *testing.T) {
+	var buf bytes.Buffer
+	enc := NewEncoder(&buf)
+	if err := enc.Encode(InvalidXMLName{}); err == nil {
+		t.Error("unexpected success")
+	} else if want := "invalid tag"; !strings.Contains(err.Error(), want) {
+		t.Errorf("error %q does not contain %q", err, want)
+	}
+}

src/encoding/xml/typeinfo.go

@@ -241,7 +241,7 @@ func lookupXMLName(typ reflect.Type) (xmlname *fieldInfo) {
 			continue
 		}
 		finfo, err := structFieldInfo(typ, &f)
-		if finfo.name != "" && err == nil {
+		if err == nil && finfo.name != "" {
 			return finfo
 		}
 		// Also consider errors as a non-existent field tag