Bug 28144: Check if issue_id defined

Historical OVERDUE fines may not have an issue_id - this causes
flooding of the logs when calling update fine

Trivial change, reading the code should be enough, but test plan below

To test:
1 - Create some fines without an issue_id
  - Either checkout an old version of Koha and add manually or
  - Checkout some items (backdated) to a patron, run fines.pl then
    UPDATE accountlines SET issue_id = NULL;
2 - Checkout a backdated item with fines in the circ rule to the same patron
3 - Run fines.pl
4 - Note the errors
5 - Apply patch
6 - Repeat and note no errors

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit bf13a36c8a)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>

C4/Overdues.pm

@@ -545,7 +545,7 @@ sub UpdateFine {
     # - accumulate fines for other items
     # so we can update $itemnum fine taking in account fine caps
     while (my $overdue = $overdues->next) {
-        if ( $overdue->issue_id == $issue_id && $overdue->status eq 'UNRETURNED' ) {
+        if ( defined $overdue->issue_id && $overdue->issue_id == $issue_id && $overdue->status eq 'UNRETURNED' ) {
             if ($accountline) {
                 $debug and warn "Not a unique accountlines record for issue_id $issue_id";
                 #FIXME Should we still count this one in total_amount ??