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

Since commit 2c9aeb9b0f
  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>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
(cherry picked from commit b004818515)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>

cataloguing/linkitem.pl

@@ -74,6 +74,9 @@ if ( $barcode && $biblionumber ) {
                 errornomodbiblio => 1
             );
         }
+        $template->param(
+            hostitemnumber => $item->itemnumber,
+        );
     }
     else {
         $template->param(
@@ -84,7 +87,6 @@ if ( $barcode && $biblionumber ) {
 
     $template->param(
         barcode        => $barcode,
-        hostitemnumber => $item->itemnumber,
     );
 
 }