Don't crash when root element is not displayed (fix #113)
Don't crash when root element is not displayed (fix #113)

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