Save currently executing lua_State in g->cur_L.

This is only a good approximation due to deficiencies in the design of
the Lua/C API. It indicates _some_ valid state that is/was executing.
Also reorder L->cframe stores to achieve a synchronously consistent state.

src/lj_ccallback.c

@@ -562,9 +562,9 @@ void LJ_FASTCALL lj_ccallback_leave(CTState *cts, TValue *o)
   }
   callback_conv_result(cts, L, o);
   /* Finally drop C frame and continuation frame. */
-  L->cframe = cframe_prev(L->cframe);
   L->top -= 2;
   L->base = obase;
+  L->cframe = cframe_prev(L->cframe);
   cts->cb.slot = 0;  /* Blacklist C function that called the callback. */
 }