Bug 12357: Fix comma issues related to additional fields.

TEST PLAN
---------
1) Apply patches, except this one.
2) prove t/db_dependent/Record.t
   -- fails like comment #38
3) Apply this patch
4) prove t/db_dependent/Record.t
   -- now it passes.
5) koha qa test tools.

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Make test work, koha-qa problems fixed in next patch

Signed-off-by: Jonathan Druart <jonathan.druart@koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>

C4/Record.pm

@@ -782,9 +782,9 @@ sub marc2bibtex {
         push @elt, qq|\t$bh[$i] = {$bh[$i+1]}|;
     }
     $tex .= join(",\n", $id, @elt);
-    $tex .= "\n";
 
     if ($additional_fields) {
+        $tex .= ",\n";
         foreach my $bibtex_tag ( keys %$additional_fields ) {
             next if $bibtex_tag eq '@';
 
@@ -802,6 +802,9 @@ sub marc2bibtex {
             }
         }
     }
+    else {
+        $tex .= "\n";
+    }
 
     $tex .= "}\n";