Bug 20829: Handle non-existent barcode when linking to host item

Bug 20829: Handle non-existent barcode when linking to host item

Since commit 2c9aeb9b0f4449d1a25274b8fbf8dae4ec11d3bf
  Bug 18277: Remove GetBiblionumberFromItemnumber - linkitem

$item is not defined if the barcode is invalid, we should not call
method on it in that case.

Test plan:
- Turn on EasyAnalyticalRecords
- Go to a detail page
- Edit > Link to host item
- Enter an invalid barcode
=> With the patch you get a friendly error message
=> Without this patch it explodes with
Can't call method "itemnumber" on an undefined value at
/home/vagrant/kohaclone/cataloguing/linkitem.pl line 86.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>