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

(now uses static variables)

lstate.c

@@ -1,5 +1,5 @@
 /*
-** $Id: lstate.c,v 2.151 2018/02/05 17:11:37 roberto Exp roberto $
+** $Id: lstate.c,v 2.152 2018/05/29 18:02:51 roberto Exp roberto $
 ** Global State
 ** See Copyright Notice in lua.h
 */
@@ -69,12 +69,11 @@ typedef struct LG {
     memcpy(b + p, &t, sizeof(t)); p += sizeof(t); }
 
 static unsigned int luai_makeseed (lua_State *L) {
-  char buff[4 * sizeof(size_t)];
+  char buff[3 * sizeof(size_t)];
   unsigned int h = cast_uint(time(NULL));
   int p = 0;
   addbuff(buff, p, L);  /* heap variable */
   addbuff(buff, p, &h);  /* local variable */
-  addbuff(buff, p, luaO_nilobject);  /* global variable */
   addbuff(buff, p, &lua_newstate);  /* public function */
   lua_assert(p == sizeof(buff));
   return luaS_hash(buff, p, h);