Bug 7138: Can't print basket group order pdf

Bug 7138: Can't print basket group order pdf

PDF print of basket groups is broken.

To test:

1) Make sure OrderPdfFormat is set to pdfformat::layout2pages or pdfformat::layout3pages
2) Create a basket with orders
3) Close the basket and create a basket group checking the checkbox
4) Print the basket group as PDF

Before patch the file is broken and when opened in an editor contains an error message.
After the patch the PDF should be generated correctly again.

Thx to Chris for helping me to fix the problem.

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>