Fix accessing deleted fire trap

melt_ice can delete the fire trap, in the case where the trap
is on ice, and a monster carrying a boulder triggers it, then drowns.

mintrap -> minliquid -> mondead -> ... -> mdrop_obj ->
   flooreffects -> boulder_hits_pool -> delfloortrap

src/trap.c

@@ -2421,7 +2421,7 @@ register struct monst *mtmp;
                 You("smell smoke.");
             if (is_ice(mtmp->mx, mtmp->my))
                 melt_ice(mtmp->mx, mtmp->my, (char *) 0);
-            if (see_it)
+            if (see_it && t_at(mtmp->mx, mtmp->my))
                 seetrap(trap);
             break;
         case PIT: