Bug 36834: (Bug 29697 follow-up) Koha explodes when trying to open in Labeled MARC...

Bug 36834: (Bug 29697 follow-up) Koha explodes when trying to open in Labeled MARC view a bibliographic record with an invalid biblionumber

After changes made in catalogue/labeledMARCdetail.pl with the bug 29697
when trying to open the Labeled MARC view with
biblionumber=<invalid_number> (e.g. a deleted biblionumber) Koha explodes
with a message << Can't call method "metadata" on an undefined value at
/kohadevbox/koha/catalogue/labeledMARCdetail.pl line 59 >>

Test plan:
==========
1. Activate the viewLabeledMARC syspref.
2. Try to open a biblio record in Labeled MARC view, giving as a biblionumber
   (in URL) a non-existing biblionumber, e.g. in ktd, with standard ktd
   test data:
   http://your_ktd:8081/cgi-bin/koha/catalogue/labeledMARCdetail.pl?biblionumber=1234567
   Koha should explode with the message:
   Can't call method "metadata" on an undefined value at
   /kohadevbox/koha/catalogue/labeledMARCdetail.pl line 59
3. Apply the patch; restart_all.
4. Repeat p. 2.  You should get a regular page with the info "The record you
   requested does not exist (1234567)".

Signed-off-by: Roman Dolny <roman.dolny@jezuici.pl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit e4e54af0e77a01d8ad1ee6ac84f5b255951f1831)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>