Fix freeing monsters not on map

Sometimes we free the monster data, but the monster is not on the
map - usually this happens if the map is full of monsters and a new one
is migrated on the level.

Make m_detach check the monster x coordinate, so it knows not to touch the map
if the monster isn't on it.

src/dog.c

@@ -464,6 +464,7 @@ fail_mon_placement:
             }
             (void) mkcorpstat(CORPSE, (struct monst *) 0, mtmp->data, xlocale,
                               ylocale, CORPSTAT_NONE);
+            mtmp->mx = mtmp->my = -1; /* for mongone, mon is not anywhere */
             mongone(mtmp);
         }
     }