Description:
------------
Currently, PDO Sqlite in memory database ignores memory_limit which is dangerous as system's memory can be exhausted by a single script.

PDO Sqlite should allocate memory using PHP allocation routines so the memory is counted and limited.

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

var_dump(memory_get_usage());
var_dump(memory_get_usage(true));
echo "\n";

for ($i = 0; $i < 3; $i++) {
    $db = new PDO('sqlite::memory:');
    $db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    $db->exec('CREATE TABLE foo (bar TEXT)');
    for ($j = 0; $j < 20; $j++) {
        $db->exec("INSERT INTO foo (bar) VALUES ('" . bin2hex(random_bytes(2 * 1024 * 1024)) . "')");
    }
    var_dump(memory_get_usage());
    var_dump(memory_get_usage(true));
    $db->exec('DROP TABLE foo');
    $db = null;
    var_dump(memory_get_usage());
    var_dump(memory_get_usage(true));
    echo "\n";
}


Expected result:
----------------
memory_get_usage includes memory allocated by sqlite and memory_limit is honored

Actual result:
--------------
int(351144)
int(2097152)

int(351536)
int(2097152)
int(351176)
int(2097152)

int(351536)
int(2097152)
int(351176)
int(2097152)

int(351536)
int(2097152)
int(351176)
int(2097152)