buglist - full level triggers impossible() from migrating mons

<email deleted> wrote:
> If more monsters fall through a trap door than can fit on the
> level below, when you go down the stairs, you get the following
> message:
>  "Program in disorder - perhaps you'd better #quit.
>  rloc(): couldn't relocate monster"
> This message seems to appear once for every monster-too-many that
> fell through the hole. I originally found this while
> intentionally completely filling a level with black puddings
> (there was a trap door I didn't know about). I also confirmed it
> in a wiz-mode test using gremlins and water.

[confirmed: moveloop -> deferred_goto -> goto_level ->
 losedogs -> mon_arrive -> rloc -> impossible]

This patch:
- causes rloc() to return TRUE if successful,
  or FALSE if it wasn't.
- adds code to mon_arrive() in dog.c to deal with
  the failed rloc()
- allows the x,y parameters to mkcorpstat() to
  be 0,0 in order to trigger random placement of the
  corpse on the level
- if you define DEBUG_MIGRATING_MONS when you build cmd.c
  then you'll have a debug-mode command #migratemons to
  store the number of random monsters that you specify
  on the migrating monsters chain.

src/wizard.c

@@ -303,7 +303,7 @@ tactics(mtmp)
 		mtmp->mavenge = 1; /* covetous monsters attack while fleeing */
 		if (In_W_tower(mtmp->mx, mtmp->my, &u.uz) ||
 			(mtmp->iswiz && !xupstair && !mon_has_amulet(mtmp))) {
-		    if (!rn2(3 + mtmp->mhp/10)) rloc(mtmp);
+		    if (!rn2(3 + mtmp->mhp/10)) (void) rloc(mtmp, FALSE);
 		} else if (xupstair &&
 			 (mtmp->mx != xupstair || mtmp->my != yupstair)) {
 		    (void) mnearto(mtmp, xupstair, yupstair, TRUE);