Fix default end index for table.concat and unpack

These were broken in commit 836a38a
which removed deprecated APIs and - accidentally - replaced the index
from which the length of the table is calculated to use the `i`
parameter as a stack index, instead of a literal "1".

CHANGELOG.md

@@ -1,8 +1,9 @@
 # Changelog
 
-## [Unreleased]
+## [v2.1]
 ### Changed
 - The `lua_Clock` typedef is now an `int64_t` to work around issues on 32-bit platforms.
+- Fixed a bug with `table.concat` and `unpack` using an incorrect default value for the optional `j` parameter.
 
 ## [v2.0]
 ### Added

liblua/lbaselib.c

@@ -328,7 +328,7 @@ static int luaB_unpack (lua_State *L) {
     int n;
     luaL_checktype(L, 1, LUA_TTABLE);
     i = luaL_optint(L, 2, 1);
-    e = luaL_opt(L, luaL_checkint, 3, (int) lua_objlen(L, i));
+    e = luaL_opt(L, luaL_checkint, 3, (int) lua_objlen(L, 1));
     if (i > e) {
         return 0; /* empty range */
     }

liblua/ltablib.c

@@ -134,7 +134,7 @@ static int table_concat (lua_State *L) {
     const char *sep = luaL_optlstring(L, 2, "", &lsep);
     luaL_checktype(L, 1, LUA_TTABLE);
     i = luaL_optint(L, 3, 1);
-    last = luaL_opt(L, luaL_checkint, 4, (int) lua_objlen(L, i));
+    last = luaL_opt(L, luaL_checkint, 4, (int) lua_objlen(L, 1));
     luaL_buffinit(L, &b);
     for (; i < last; i++) {
         addfield(L, &b, i);