fix "Undefined offset" notice in _applyGSUBrulesIndic()

[SennoYuki]

reproduce this bug

<?php
require ('./vendor/autoload.php');

$model = [];
$origin = ['user_name'=>'✿્᭄͜͡'];
$model['user_name'] = htmlspecialchars($origin['user_name']);
$mpdf = new Mpdf\Mpdf(['format' => 'A4']);
$mpdf->autoScriptToLang = true;
$mpdf->autoLangToFont = true;
$mpdf->WriteHTML(getOutput($model));
$mpdf->output('1.pdf');

function getOutput($params) {
  $html = <<<EOT
<table>
  <tr>
    <td colspan=2 height="20" align="left" valign=bottom><font face="Times New Roman" color="#000000">{$params['user_name']}</font></td>
  </tr>
</table>
EOT;
  return $html;
}

[finwe]

Use strict comparison here, no reason not to.

[SennoYuki]

Use strict comparison here, no reason not to.

I have modified that.

[finwe]

Thanks for the PR. Please add a test case reproducing your change. Can you also please explain the change a bit?

[SennoYuki]

Thanks for the PR. Please add a test case reproducing your change. Can you also please explain the change a bit?

Do I need create a issue for the test?

[finwe]

Just amend the test case to this PR. See tests/Issues dir for reference

[SennoYuki]

Just amend the test case to this PR. See tests/Issues dir for reference

I have added the unit test file. Do I need do anything eles?

[SennoYuki]

Just amend the test case to this PR. See tests/Issues dir for reference

Look at this please. Thanks