encoding/xml: EncodeToken silently eats tokens with invalid type

EncodeToken takes a Token (i.e. an interface{}) as a parameter,
and expects a value of type StartElement, EndElement, CharData,
Comment, ProcInst, or Directive.

If a pointer is passed instead, or any type which does not match
this list, the token is silently ignored.

Added a default case in the type switch to issue a proper error
when the type is invalid.

The behavior could be later improved by allowing pointers to
token to be accepted as well, but not for go1.5.

Fixes #11719

Change-Id: Ifd13c1563450b474acf66d57669fdccba76c1949
Reviewed-on: https://go-review.googlesource.com/12252
Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/encoding/xml/marshal.go

@@ -199,6 +199,7 @@ var (
 // elements (including the StartElement itself) will use the declared
 // prefix when encoding names with matching namespace URIs.
 func (enc *Encoder) EncodeToken(t Token) error {
+
 	p := &enc.p
 	switch t := t.(type) {
 	case StartElement:
@@ -245,6 +246,9 @@ func (enc *Encoder) EncodeToken(t Token) error {
 		p.WriteString("<!")
 		p.Write(t)
 		p.WriteString(">")
+	default:
+		return fmt.Errorf("xml: EncodeToken of invalid token type")
+
 	}
 	return p.cachedWriteError()
 }