Don't crash when relative boxes have absolute children (fix #118)
Don't crash when relative boxes have absolute children (fix #118)

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