Fix `instance_to_array` behavior on recursive classes

[DrDet]

This code leads to SIGSEGV error with the following message "Error -1: Callstack overflow" now.

<?php

class A {
    /** @var A */
    public $this = null;

    public function __construct() {
        $this->this = $this;
    }
}

function demo() {
    $a = new A;
    var_dump(instance_to_array($a));
}

demo();

relates to KPHP-1530