Batch import fails if all imported publications are duplicates

To reproduce, try to import twice the same file (I tried with biblio_pm.module), the second time the batch will fall in an infinite loop. Logs will show Warning: Division by zero in biblio_import_batch_operations() (line 299 of sites/all/modules/biblio/includes/biblio.import.export.inc).

This is because in the case of all duplicates, there are zero elements in the cache and therefore $context['sandbox']['max'] == 0.