LP#1970486: fix SuperCat crash on serial units with stat cats

LP#1970486: fix SuperCat crash on serial units with stat cats

This patch fixes a bug where retrieving a title via SuperCat (in
particular, with holdings XML) can crash of the title has a serial
unit (i.e., barcoded serial item) attached that also has a statistical
category.

To test
-------
[1] Create a title with a barcoded serial item.
[2] Attempt to fetch the title via SuperCat:

https://EGSERVER/opac/extras/supercat/retrieve/marcxml-full/record/BIBID

[3] Note that an internal server error is returned.
[4] Apply the patch and repeat step 2. This time, an XML
    document should be returned that includes the serial item
    and its stat cat.

Signed-off-by: Galen Charlton <gmc@equinoxOLI.org>
Signed-off-by: Mike Rylander <mrylander@gmail.com>