Bug 31873: Check ->find before calling ->safe_delete

Bug 31873: Check ->find before calling ->safe_delete

When there is no item, you cannot safely delete it :)

Resolve this warning:
  Can't call method "safe_delete" on an undefined value at /usr/share/koha/cataloguing/additem.pl line 379
(Line numbers may vary across versions.)

Test plan:
Open item editor.
Add a new item.
Add another tab with same form. Delete new item.
Go back to former tab. Try to delete again.
You should see an alert now on top of the form.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>