Bug 16750: Redirect from selectbranchprinter.pl to additem.pl causes software error

Bug 16750: Redirect from selectbranchprinter.pl to additem.pl causes software error

How to reproduce:
1) Edit and save an item in the items editor
2) You should be redirected back to the items editor, note that the
biblionumber is passed in the url
3) Choose "set library" from the upper right hand pulldown on the page
4) Choose a different library, click submit
5) You should get the error message:
Software error:

Can't call method "fields" on an undefined value at
/home/vagrant/kohaclone/cataloguing/additem.pl line 736.

Test Plan:
1) Apply this patch
2) Follow plan above
3) No software error should occur at step 5

Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
(cherry picked from commit 7d7c8c12e70295f4dd15a3435235c81539c333f5)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>