Bug 7817 - Items Permanent location (shelving location) is set to NULL when item...

Bug 7817 - Items Permanent location (shelving location) is set to NULL when item is edited

A quick fix to prevent more damages.
No perceived side-effects so far.

Signed-off-by: David Cook <dcook@prosentient.com.au>
This is actually a perfectly good fix for this issue. I've changed
the explanatory comment to explain why.

Another option would be to remove the 'exists' check in the sub
_do_column_fixes_for_mod(), but this is just as functional.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Detailed comment on Bugzilla.
Adding a small follow-up.

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>