no more 'luaO_nilobject' to avoid comparison of global variable addresses

(now uses static variables)

lobject.h

@@ -1,5 +1,5 @@
 /*
-** $Id: lobject.h,v 2.142 2018/04/04 14:23:41 roberto Exp roberto $
+** $Id: lobject.h,v 2.143 2018/06/01 16:51:34 roberto Exp roberto $
 ** Type definitions for Lua objects
 ** See Copyright Notice in lua.h
 */
@@ -150,10 +150,6 @@ typedef StackValue *StkId;  /* index to stack elements */
 #define setnilvalue(obj) settt_(obj, LUA_TNIL)
 
 
-/* (address of) a fixed nil value */
-#define luaO_nilobject		(&luaO_nilobject_)
-
-
 /*
 ** Variant tag, used only in tables to signal an empty slot
 ** (which might be different from a slot containing nil)
@@ -730,8 +726,6 @@ typedef struct Table {
 #define sizenode(t)	(twoto((t)->lsizenode))
 
 
-LUAI_DDEC const TValue luaO_nilobject_;
-
 /* size of buffer for 'luaO_utf8esc' function */
 #define UTF8BUFFSZ	8