encoding/asn1: fix panic when Marshaling nil.

Fixes #11127.

Change-Id: Ibcfc3a05e91fa4260d70b04bee2bbba2376bd313
Reviewed-on: https://go-review.googlesource.com/13923Reviewed-by: Adam Langley <agl@golang.org>

src/encoding/asn1/asn1_test.go

@@ -940,3 +940,15 @@ func TestUnmarshalInvalidUTF8(t *testing.T) {
 		t.Fatalf("Expected error to mention %q but error was %q", expectedSubstring, err.Error())
 	}
 }
+
+func TestMarshalNilValue(t *testing.T) {
+	nilValueTestData := []interface{}{
+		nil,
+		struct{ v interface{} }{},
+	}
+	for i, test := range nilValueTestData {
+		if _, err := Marshal(test); err == nil {
+			t.Fatal("#%d: successfully marshaled nil value", i)
+		}
+	}
+}

src/encoding/asn1/marshal.go

@@ -506,6 +506,9 @@ func marshalBody(out *forkableWriter, value reflect.Value, params fieldParameter
 }
 
 func marshalField(out *forkableWriter, v reflect.Value, params fieldParameters) (err error) {
+	if !v.IsValid() {
+		return fmt.Errorf("asn1: cannot marshal nil value")
+	}
 	// If the field is an interface{} then recurse into it.
 	if v.Kind() == reflect.Interface && v.Type().NumMethod() == 0 {
 		return marshalField(out, v.Elem(), params)