Added Undefined offset test for reading XmlDataSet

If you define n columns and add n + 1 rows it will generate an undefined offset error.


Example .xml file:

<?xml version="1.0" encoding="UTF-8"?>
<dataset>
    <table name="drop_chat_file_repository">
        <column>id</column>
        <row>
            <value>1</value>
            <value>1</value>
        </row>
    </table>
</dataset>

Therefore a check is necessary. Because $rowElement->children()->count() didn't work for me properly it was necessary to test the index in the foreach instead of testing it beforehand with something like 'if ($rowElement->children()->count() >= count($tableInstanceColumns) {}
Update XmlDataSet.php
Merge pull request #122 from Sicaine/patch-1

Added Undefined offset test for reading XmlDataSet