Ensure that vision is not used during done()

(and panic() which calls done())

This uses the flag that <Someone> recently added.

At least one crash in the past was caused by

? -> panic() -> done(11) -> * vision_recalc(2) -> newsym() -> crash!
if  u.ux and u.uy are 0

src/end.c

@@ -582,8 +582,8 @@ die:
 	program_state.gameover = 1;
 	/* in case of a subsequent panic(), there's no point trying to save */
 	program_state.something_worth_saving = 0;
-	/* turn off vision subsystem */
-	vision_recalc(2);
+	/* render vision subsystem inoperative */
+	iflags.vision_inited = 0;
 	/* might have been killed while using a disposable item, so make sure
 	   it's gone prior to inventory disclosure and creation of bones data */
 	inven_inuse(TRUE);