Fixed initialization of global variables

When calling 'luaK_storevar', the 'expdesc' for the variable must be
created before the one for the expression, to satisfy the assumptions
for register allocation. So, in a statement like 'global a = exp', where
'a' is actually '_ENV.a', this variable must be handled before the
initializing expression 'exp'.

lcode.c

@@ -1242,7 +1242,7 @@ static void codenot (FuncState *fs, expdesc *e) {
 ** Check whether expression 'e' is a short literal string
 */
 static int isKstr (FuncState *fs, expdesc *e) {
-  return (e->k == VK && !hasjumps(e) && e->u.info <= MAXARG_B &&
+  return (e->k == VK && !hasjumps(e) && e->u.info <= MAXINDEXRK &&
           ttisshrstring(&fs->f->k[e->u.info]));
 }