Bug fix: sometimes box.children[i].parent was not box.
Bug fix: sometimes box.children[i].parent was not box.

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