Fix negative values in counters.
Fix negative values in counters.

marclaporte/WeasyPrint,prepare/TestWeasyPrint,andrewleech/WeasyPrint,andrewleech/WeasyPrint,marclaporte/WeasyPrint,prepare/TestWeasyPrint,jasco/WeasyPrint,Kozea/WeasyPrint,Kozea/WeasyPrint,jasco/WeasyPrint