Description:
------------
After the undef index notice is thrown, we still assume that the variable is an array, even though the type might have changed in the meantime.

Not really sure what we can do about this, especially when we're dealing with a deeply nested array that has been destroyed.

Test script:
---------------
<?php
$my_var = null;
set_error_handler(function() use(&$my_var) {
    $my_var = 0;
});
$my_var[0] .= "xyz";


Actual result:
--------------
php: /home/nikic/php-7.2/Zend/zend_hash.c:712: _zend_hash_index_add_or_update_i: Assertion `((ht)->gc.refcount == 1) || ((ht)->u.flags & (1<<6))' failed.
Aborted
nikic@MUNIT-271:~/php-7.2$ vim t036.php