FIX: Fix a bug in bitset.h

[zhanglei1949]

As titled.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 60.92%. Comparing base (1f34cc1) to head (cc9b99a).

Additional details and impacted files

[](https://app.codecov.io/gh/alibaba/libgrape-lite/pull/159?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=alibaba) ```diff @@ Coverage Diff @@ ## master #159 +/- ## ======================================= Coverage 60.92% 60.92% ======================================= Files 70 70 Lines 6652 6652 ======================================= Hits 4053 4053 Misses 2599 2599 ``` | [Flag](https://app.codecov.io/gh/alibaba/libgrape-lite/pull/159/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=alibaba) | Coverage Δ | | |---|---|---| | [app_tests](https://app.codecov.io/gh/alibaba/libgrape-lite/pull/159/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=alibaba) | `60.92% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=alibaba#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files](https://app.codecov.io/gh/alibaba/libgrape-lite/pull/159?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=alibaba) | Coverage Δ | | |---|---|---| | [grape/utils/bitset.h](https://app.codecov.io/gh/alibaba/libgrape-lite/pull/159?src=pr&el=tree&filepath=grape%2Futils%2Fbitset.h&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=alibaba#diff-Z3JhcGUvdXRpbHMvYml0c2V0Lmg=) | `75.00% <ø> (ø)` | |