fix stream while max_buf_size < 0

[chenbay]

What problem does this PR solve?

fix stream logic while options.max_buf_size < 0

_cur_buf_size is size_t, options.max_buf_size is int. options.max_buf_size 赋值 会导致错误的含义。

Issue Number:

Problem Summary:

What is changed and the side effects?

Changed:

Side effects:

• Performance effects(性能影响):

• Breaking backward compatibility(向后兼容性):

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Check List:

• Please make sure your changes are compilable(请确保你的更改可以通过编译).
• When providing us with a new feature, it is best to add related tests(如果你向我们增加一个新的功能, 请添加相关测试).
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[chenbay]

@wwbmmm 帮忙看看，之前feat的一个问题。

[wwbmmm]

LGTM