Prevent unnecessary DB access in add_list_member

In add_list_member the value $current_list_members_count is discarded if max_list_member == 0 (default setting).
This prevents the "high db load" produced by the successive DB accesses.

src/lib/Sympa/List.pm

@@ -3822,7 +3822,10 @@ sub add_list_member {
     $self->{'add_outcome'}{'remaining_members_to_add'} =
         $self->{'add_outcome'}{'expected_number_of_added_users'};
 
-    my $current_list_members_count = $self->get_total;   # FIXME: high db load
+    my $current_list_members_count = 0;
+    if ($self->{'admin'}{'max_list_members'} > 0) {
+        $current_list_members_count = $self->get_total;   # FIXME: high db load
+    }
 
     my $sdm = Sympa::DatabaseManager->instance;