added empty token check and copyright

jwt.go

@@ -1,3 +1,5 @@
+// Copyright (C) 2018 Marius Schellenberger
+
 // Package jwt provides a easy to use JSON Web Token and blacklisting library
 package jwt
 
@@ -24,6 +26,7 @@ const (
 
 var (
 	ErrNoJWT               = errors.New("not a json web token")
+	ErrEmptyToken          = errors.New("token is empty")
 	ErrUnsupportedAlg      = errors.New("unsupported algorithm")
 	ErrInvalid             = errors.New("token validation failed")
 	ErrBlacklisted         = errors.New("token blacklisted")
@@ -274,6 +277,9 @@ func (jwt *JWT) Stop() error {
 
 // DecodeToken decodes a raw string token into a *Token object
 func DecodeToken(token string) (t *Token, err error) {
+	if token == "" {
+		return nil, ErrEmptyToken
+	}
 	parts := strings.Split(token, ".")
 	if len(parts) < 3 {
 		return nil, ErrMissingTokenParts