[BUG]使用fastjson2字符串转对象出错Index 1034144 out of bounds for length 1034144

### 问题描述
使用 JSON.parseObject(sJson);把json转换为对象的时候出现了错误如下：
"java.lang.Exception: java.lang.ArrayIndexOutOfBoundsException: Index 1034144 out of bounds for length 1034144 com.alibaba.fastjson2.JSONReaderUTF16.readNull(JSONReaderUTF16.java:4981)com.alibaba.fastjson2.JSONReader.readObject(JSONReader.java:2315)com.alibaba.fastjson2.JSONReader.readArray(JSONReader.java:2548)com.alibaba.fastjson2.JSONReader.read(JSONReader.java:2116)com.alibaba.fastjson2.JSON.parseObject(JSON.java:244)
注：这个json文件1.91MB

### 环境信息

- OS信息： [e.g.：CentOS 8.4.2105 4Core 3.10GHz 16 GB]
- JDK信息： [e.g.：Openjdk 14]
- 版本信息：[e.g.：Fastjson2 2.0.39]

### 重现步骤
超大的json文件肯定出错。

### 期待的正确结果
能支持大文件转对象

### 相关日志输出
"java.lang.Exception: java.lang.ArrayIndexOutOfBoundsException: Index 1034144 out of bounds for length 1034144 com.alibaba.fastjson2.JSONReaderUTF16.readNull(JSONReaderUTF16.java:4981)com.alibaba.fastjson2.JSONReader.readObject(JSONReader.java:2315)com.alibaba.fastjson2.JSONReader.readArray(JSONReader.java:2548)com.alibaba.fastjson2.JSONReader.read(JSONReader.java:2116)com.alibaba.fastjson2.JSON.parseObject(JSON.java:244)

Start by reproducing JSON.parseObject(sJson) with a JSON input larger than 1.91 MB, then trace the reported stack through JSON.java, JSONReader.readArray/readObject, and JSONReaderUTF16.java at line 4981. Done means the large JSON file is converted to an object without the reported ArrayIndexOutOfBoundsException.

Written by the indexing model from the issue text.