Description:
------------
Throwing from jsonSerialize() crashes on PHP 7.1

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

class JsonSerializableObject implements \JsonSerializable
{
    public function jsonSerialize()
    {
        throw new \Exception('This error is expected');
    }
}

$obj = new JsonSerializableObject();

echo json_encode($obj);
echo "\n";


Expected result:
----------------
an exception

Actual result:
--------------
a segfault