make corpse visible if stethoscope told you about it

make corpse visible if stethoscope told you about it

Bug bz14, no web id.

Steps to reproduce:
- have a stethoscope handy.
- place an amulet in a doorway and move one square outside the room's door.
- create and lead an invisible stalker to be on top of amulet, with you
  just outside the room beside the door square.
- zap the stalker asleep with a wand of sleep.
- put on a blindfold.
- quaff a potion of object detection.
- amulet shows in the doorway.
- save the game and keep the savefile for ease of returning to this point.

Bug 1 observed (remember that you're blind due to blindfold):
- zap a wand of death at the stalker that you know to be on top of
the amulet, but that the game gives no indication of.
- if the stalker left a corpse, and you apply a stethoscope to the
  doorway, the game tells you that "You determine that that unfortunate
  being is dead" yet no being or corpse is displayed, still just the amulet.

Fix that by calling map_object(corpse, TRUE) in its_dead() under these circumstances.

The circumstances in the original report were also reproduced, specifically:
If a stethoscope finds an unseen monster on a square with an
object-detected object while blind, after killing the monster, the
object isn't remembered.

That remains unfixed because the I (invis monster glyph) aleady overwrote the
detected object glyph, so it is a much tougher situation.