FIX: Fix a bug in bitset.h

[zhanglei1949]

As titled.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #159   +/-   ##
=======================================
  Coverage   60.92%   60.92%           
=======================================
  Files          70       70           
  Lines        6652     6652           
=======================================
  Hits         4053     4053           
  Misses       2599     2599           

Flag	Coverage Δ
app_tests	60.92% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
grape/utils/bitset.h	75.00% <ø> (ø)