Lua: fix previous commit (83ab5425)

modules/misc/lua/libs/dialog.c

@@ -420,7 +420,7 @@ static int vlclua_dialog_add_button( lua_State *L )
     p_widget->psz_text = strdup( luaL_checkstring( L, 2 ) );
     lua_settop( L, 10 );
     lua_pushlightuserdata( L, p_widget );
-    lua_insert( L, 10 );
+    lua_pushvalue( L, 3 );
     lua_settable( L, LUA_REGISTRYINDEX );
     p_widget->p_sys = NULL;