display glitch: warning vs remembered,unseen

display glitch: warning vs remembered,unseen

Noticed while testing the fix for the recently reported clairvoyance
bug.  I saw a '1' move onto an 'I', then when it moved again the 'I'
reappeared.  The remembered unseen monster couldn't be there anymore
if the warned-of monster was able to walk through that spot, so
remove any 'I' when showing a warning (digit) to stop remembering an
unseen monster at the warning spot.

Nobody has ever reported this so fixing it isn't urgent, but fixing
it is trivial so I'm doing it in now (without the clairvoyance fix).