NullPointer insertSheet with exceeding index fix

[CSendner]

Nullpointer with this sequence:

1. Create new document
2. Insert at exceeding index (i.e. 1 with a new document)
3. Try to write to document
4. Crash while writing, because internal structure has wrong index.

    QXlsx::Document xlsx(file_url);
    if(!xlsx.insertSheet(1, "records")){
        qDebug() << "Never here";
    }
    xlsx.write(1, 1, "id");//crash
    xlsx.save();

[VSRonin]

LGTM I'll give it a couple of days in case someone wants to object otherwise I'll merge