Description:
------------
You can easily create a segfault in jsonSerialize() by returning a new instance of self. You can see this affects all actively supported versions of PHP here: https://3v4l.org/tLMv6

I'm working on a patch and will be submitting it as a PR soon. :)

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

class Foo implements JsonSerializable {
  public function jsonSerialize() {
    return new self;
  }
}

var_dump(json_encode(new Foo));


Expected result:
----------------
We'd should see a fatal error raised on an exception thrown.

Actual result:
--------------
Segmentation fault. Doh!