Description:
------------
ReflectionParameter::getDefaultValue() should not trigger uncatchable (not even in PHP 7) fatal error, when constant is undefined. It should throw ReflectionException or Error.

Test: https://3v4l.org/X8gXu and https://3v4l.org/EEQFI

Test script:
---------------
<?php

class Test
{
    function method($a = self::UNDEFINED) // or UndefinedClass::UNDEFINED
    {
    }
}

$rp = new ReflectionParameter(['Test', 'method'], 'a');
$val = $rp->getDefaultValue();

Expected result:
----------------
ReflectionException or Error in PHP 7

Actual result:
--------------
Fatal Error