Reported directly to devteam, mounted hero whose steed got hit
for knockback effect triggered impossible "no monster to remove".
In addition to fixing that, this makes a knockback attempt at a
hero who is stuck to a cursed saddle knock the hero and steed back
instead of knocking the hero out of the saddle.
mhurtle_step() should be able to use u.ux0,u.uy0 to update the
hero's old location after moving the hero in order to move the
steed, but the value was different from what was expected and the
map showed stale steed symbol when I used that. I'm not sure what
is going on there; saving u.ux,u.uy before moving the hero worked
as intended so I didn't pursue it.
a1dae3caf8