Some map border related fixes

1. Check for entity addition success in spawn_item implementation
2. Check for success in item_drop callback, so that the player
doesn't lose the item if they are outside bounds and try to drop it.
3. When existing player joins game, check that their position is inside
map bounds. If not, set their position to the return value of findSpawnPos().
4. Make findSpawnPos() respect the border

2 fixes a lua crash if a player drops an item outside map bounds.
3 fixes an assertion crash if a player leaves when being outside map bounds,
and then rejoins.

builtin/game/item.lua

@@ -349,12 +349,16 @@ function core.item_drop(itemstack, dropper, pos)
 			v.y = v.y*2 + 2
 			v.z = v.z*2
 			obj:setvelocity(v)
+			return itemstack
 		end
 
 	else
-		core.add_item(pos, itemstack)
+		if core.add_item(pos, itemstack) then
+			return itemstack
+		end
 	end
-	return itemstack
+	-- If we reach this, adding the object to the
+	-- environment failed
 end
 
 function core.do_item_eat(hp_change, replace_with_item, itemstack, user, pointed_thing)