LoadField如果输入的是负数，应该转为正数位置

Assets/XLua/Src/Utils.cs

@@ -28,6 +28,7 @@ namespace XLua
     {
         public static bool LoadField(RealStatePtr L, int idx, string field_name)
         {
+            idx = idx > 0 ? idx : LuaAPI.lua_gettop(L) + idx + 1;// abs of index
             LuaAPI.xlua_pushasciistring(L, field_name);
             LuaAPI.lua_rawget(L, idx);
             return !LuaAPI.lua_isnil(L, -1);