Fix memory leak in pdo_parse_params

Fixed memory leak after executing PDOStatement::execute() with bound parameters. Also removed unnecessary copying of result query string.

Test case (I don't know whether it can be tested in Travis because it requires MySQL connection):
```php
<?php
$pdo = new PDO('mysql:dbname=test;host=localhost', 'root', '');
$data = str_repeat('a', 1000000);

for ($n = 0; $n < 50; $n++) {
    $stmt = $pdo->prepare('SELECT :data = 1');
    $stmt->bindValue(':data', $data);
    $stmt->execute();
    $stmt = null;
    echo sprintf("%dM\n", memory_get_usage(true) / 1048576);
}
```