'constfolding' passes a proper Lua state to 'luaO_arith'

[Daurnimator <quae@...>]

  - constant folding (in lcode.c) uses luaO_arith with a result on the C stack
  - luaO_arith falls back to metamethods using luaT_trybinTM.
  - luaT_trybinTM assumes that the result goes onto the Lua stack

To help catch errors in this arrangement, there is an assert in luaO_arith:
  lua_assert(L != NULL);  /* should not fail when folding (compile time) */

However, in this commit
https://github.com/lua/lua/commit/99ac4a260fc1bf958515c1816d866852194493f2
constant folding now passes a lua_State* to luaO_arith.
I don't think this is desired: should that commit be backed out?