Condition to check if a utf8 string is malformed is wrong

In the class type_cast_helper there is a condition to check if a utf8 string is malformed

                    if (!preg_match('/^./u', $result))

                    {

                        $result = '';

                    } 

But is causing false positives with some inputs.

Possible solutions:

https://stackoverflow.com/questions/6723562/how-can-i-detect-a-malformed-utf-8-string-in-php