Well, yeah, Quirrell could have considered the possibility that Harry would do something he couldn't have possibly planned for. But considering the possibility isn't the same as planning for it, and almost by definition he couldn't have planned for it. If you look at the passage just before the fight with Bahry, he did maneuver Harry into a position where even if he did have some spell that could block an Avada, he wouldn't have been able to direct it into the fight in time. He was expecting some interference, and he planned for it so that Harry wouldn't be able or would feel inclined not to interfere.