fix array offset access with curly braces deprecation

[jglhelias]

Deprecated: Array and string offset access syntax with curly braces is deprecated in wordpress\wp-content\plugins\sqlite-integration\pdoengine.class.php on line 788
Deprecated: Array and string offset access syntax with curly braces is deprecated in wordpress\wp-content\plugins\sqlite-integration\pdoengine.class.php on line 791

php 7.4

$ php --version
PHP 7.4.11 (cli) (built: Sep 29 2020 13:17:42) ( NTS Visual C++ 2017 x64 )
Copyright (c) The PHP Group
Zend Engine v3.4.0, Copyright (c) Zend Technologies
    with Zend OPcache v7.4.11, Copyright (c), by Zend Technologies
    with Xdebug v2.9.8, Copyright (c) 2002-2020, by Derick Rethans