discard migrating monsters when entering endgame

When the hero enters the planes branch, all the rest of the dungeon
gets discarded since it can no longer be reached.  At the time that
that takes place, throw away any migrating monsters waiting to arrive
on any of those levels.

src/do.c

@@ -1463,6 +1463,8 @@ goto_level(
         /* mark #overview data for all dungeon branches as uninteresting */
         for (l_idx = 0; l_idx < g.n_dgns; ++l_idx)
             remdun_mapseen(l_idx);
+        /* get rid of monsters scheduled to migrate to discarded levels */
+        discard_migrating_mons();
     }
 
     if (Is_rogue_level(newlevel) || Is_rogue_level(&u.uz))