When objp is nullptr, ai_door_is_openable assumes Buddy_objnum is a
valid object number, but this assumption is not guaranteed to be true.
When it is false, the game crashes. This crash can be triggered by a
brain robot trying to make nearby robots snipe.
Fix the crash by passing the robot of interest.
Reported-by: Yarn <http://www.dxx-rebirth.com/frm/index.php/topic,2165.0.html>