fix for #21: segfault on going downstairs

game.cpp

@@ -2041,6 +2041,7 @@ void game::debug()
     z.clear();
     levx = tmp.x * 2 - int(MAPSIZE / 2);
     levy = tmp.y * 2 - int(MAPSIZE / 2);
+    set_adjacent_overmaps(true);
     m.load(this, levx, levy);
    }
   } break;
@@ -4426,6 +4427,7 @@ void game::examine()
   //m.save(&cur_om, turn, levx, levy);
   overmap(this, cur_om.posx, cur_om.posy, -1);
   cur_om = overmap(this, cur_om.posx, cur_om.posy, cur_om.posz + movez);
+  set_adjacent_overmaps(true);
   m.load(this, levx, levy);
   update_map(u.posx, u.posy);
   for (int x = 0; x < SEEX * MAPSIZE; x++) {
@@ -6908,6 +6910,7 @@ void game::vertical_move(int movez, bool force)
  cur_om.save(u.name);
  //m.save(&cur_om, turn, levx, levy);
  cur_om = overmap(this, cur_om.posx, cur_om.posy, cur_om.posz + movez);
+ set_adjacent_overmaps(true);
  map tmpmap(&itypes, &mapitems, &traps);
  tmpmap.load(this, levx, levy, false);
  cur_om = overmap(this, cur_om.posx, cur_om.posy, original_z);

game.h

@@ -237,6 +237,7 @@ class game
   WINDOW *w_messages;
   WINDOW *w_location;
   WINDOW *w_status;
+  overmap *om_hori, *om_vert, *om_diag; // Adjacent overmaps
 
  private:
 // Game-start procedures
@@ -388,7 +389,6 @@ class game
 
   calendar nextspawn; // The turn on which monsters will spawn next.
   calendar nextweather; // The turn on which weather will shift next.
-  overmap *om_hori, *om_vert, *om_diag; // Adjacent overmaps
   int next_npc_id, next_faction_id, next_mission_id; // Keep track of UIDs
   std::vector <game_message> messages;   // Messages to be printed
   int curmes;	  // The last-seen message.

map.cpp

@@ -2784,11 +2784,28 @@ bool map::loadn(game *g, int worldx, int worldy, int gridx, int gridy, bool upda
 //  squares divisible by 2.
   int newmapx = worldx + gridx - ((worldx + gridx) % 2);
   int newmapy = worldy + gridy - ((worldy + gridy) % 2);
+  overmap* this_om = &(g->cur_om);
+
+  // slightly out of bounds? to the east, south, or both?
+  // cur_om is the one containing the upper-left corner of the map
+  if (newmapx >= OMAPX*2){
+     newmapx -= OMAPX*2;
+     this_om = g->om_hori;
+     if (newmapy >= OMAPY*2){
+        newmapy -= OMAPY*2;
+        this_om = g->om_diag;
+     }
+  }
+  else if (newmapy >= OMAPY*2){
+     newmapy -= OMAPY*2;
+     this_om = g->om_vert;
+  }
+
   if (worldx + gridx < 0)
    newmapx = worldx + gridx;
   if (worldy + gridy < 0)
    newmapy = worldy + gridy;
-  tmp_map.generate(g, &(g->cur_om), newmapx, newmapy, int(g->turn));
+  tmp_map.generate(g, this_om, newmapx, newmapy, int(g->turn));
   return false;
  }
  return true;