Soundness of table.sort

[takuya hashimoto <takuya.hashimoto@...>]

  function comp(l, r) return l <= r end
  list = {1,3,2,4}
  table.sort(list, comp)

`comp` is an invalid order function for sorting,
but no error occurs, and `list` become {1,2,4,3}.
I think this is not a sound algorithm.
By introducing the patch below, an error will occur as expected.

--- lua-5.3.1/src/ltablib.c     2015-11-04 02:44:14.099666336 +0000
+++ lua-5.3.1-mod/src/ltablib.c 2015-11-04 03:55:51.602169624 +0000
@@ -289,7 +289,7 @@
     for (;;) {  /* invariant: a[l..i] <= P <= a[j..u] */
       /* repeat ++i until a[i] >= P */
       while ((*ta->geti)(L, 1, ++i), sort_comp(L, -1, -2)) {
-        if (i>=u) luaL_error(L, "invalid order function for sorting");
+        if (i>=u-1) luaL_error(L, "invalid order function for sorting");
         lua_pop(L, 1);  /* remove a[i] */
       }
       /* repeat --j until a[j] <= P */