Description:
------------
Roundcube Webmail code worked on RC3, since I updated to RC4 (ubuntu packages from deb.sury.org) it gives 100% reproduceable "zend_mm_heap corrupted" errors. Sometimes it gives memory access violation (core dumped), but not always.

Test script:
---------------
I tried create a simple script, but couldn't reproduce without including the Roundcube Framework. So, to reproduce clone git@github.com:roundcube/roundcubemail.git repository and put following script inside:

<?php
require_once 'program/include/iniset.php';
$RCMAIL = rcmail::get_instance();
echo "it works";
?>

and execute it.

Expected result:
----------------
it works

Actual result:
--------------
zend_mm_heap corrupted