feeling monsters while blind and levitating

<Someone> reported this during 3.4.0 beta.  If you are blind and levitating,
unseen monsters are shown as "I", just like when not levitating, but they
are never erased after they move, unlike not levitating.  Display this
case correctly as well.

doc/fixes34.1

@@ -131,6 +131,7 @@ headstone writing was using the adjective "weird" when engraving with a wand
 	of digging.
 don't report "you were riding" if you die as a result of dismounting
 allow #untrapping of chests that are co-located with floor traps and hero
+unmap "I" symbols when searching while blind and levitating
 
 
 Platform- and/or Interface-Specific Fixes

src/display.c

@@ -536,7 +536,8 @@ feel_location(x, y)
 	     * underneath if already seen.  Otherwise, show the appropriate
 	     * floor symbol.
 	     *
-	     * Similarly, if the hero digs a hole in a wall.  In this case,
+	     * Similarly, if the hero digs a hole in a wall or feels a location
+	     * that used to contain an unseen monster.  In these cases,
 	     * there's no reason to assume anything was underneath, so
 	     * just show the appropriate floor symbol.  If something was
 	     * embedded in the wall, the glyph will probably already
@@ -557,8 +558,9 @@ feel_location(x, y)
 					       cmap_to_glyph(S_stone);
 		    show_glyph(x,y,lev->glyph);
 		}
-	    } else if (lev->glyph >= cmap_to_glyph(S_stone) &&
-		       lev->glyph < cmap_to_glyph(S_room)) {
+	    } else if ((lev->glyph >= cmap_to_glyph(S_stone) &&
+			lev->glyph < cmap_to_glyph(S_room)) ||
+		       glyph_is_invisible(levl[x][y].glyph)) {
 		lev->glyph = lev->waslit ? cmap_to_glyph(S_room) :
 					   cmap_to_glyph(S_stone);
 		show_glyph(x,y,lev->glyph);