Fix #H4411: Targeting detected monster through wall with fireball

Fix #H4411: Targeting detected monster through wall with fireball

Fireball and cone of cold could target detected monsters
through a wall when cast at skilled or higher.  This allowed
eg. targeting the Wiz from outside his tower.

Use walk_path to determine the actual location where the
spell will hit, so trying to cast through a wall will
make the explosion happen at the nearest empty space.