wand bouncing shouldn't try to bounce through unexplored tiles

[doy]

Right now it assumes that unexplored tiles are empty, but they could also be rock, which changes where the bounce goes. For instance:

    ----
    .m. 
   -@   
##      
@       
#       

TAEB thought that firing a magic missile northeast would hit the mimic (because it assumed that the unexplored tile two spaces to the left of the mimic was empty), but instead it hit the shk.

This isn't an issue for non-bounced shots, since if we can see a tile and it's in range of a non-bounced shot, there can't be any walls in between.