Fix possible segfault for cvars registered from lua with flag CV_CALL

(made same PR to "official" kart repository but it doesn't seem it would get merged any time soon)

So, the problem was that lua_next doesn't guarantee order of keys when iterating through table, so sometimes CV_RegisterVar checks field func before field flags, doesn't add it because cvar doesn't have CV_CALL flag, and at the end it has CV_CALL flag but no valid func, and then modifying this cvar causes game to crash.