don't empty the error message

refer from the menu: [htmlspecialchars](http://cn2.php.net/htmlspecialchars#refsect1-function.htmlspecialchars-returnvalues)
> If the input string contains an invalid code unit sequence within the given encoding an empty string will be returned
Merge pull request #512 from Yahasana/patch-1

Don't empty the error message when there is an invalid code unit sequence