Caching pointers to Lua objects - don't!

[whoward69]

From ProductionPopup.lua (hotfix 1.0.3.70 change)

-- NOTE: Storing the PlayerID/CityID for the city, it is not safe to store the pointer to a city
--       over multiple frames, the city could get removed by the game core thread
local g_currentCityOwnerID = -1;
local g_currentCityID = -1;