[0.7] Fix incorrect assertion in `BM_VecSimBasics::UpdateAtBlockSize` benchmark

[meiravgri]

backport #819 to 0.7

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 95.90%. Comparing base (dfd71f9) to head (0da50c0).

Additional details and impacted files

@@            Coverage Diff             @@
##              0.7     #825      +/-   ##
==========================================
+ Coverage   95.68%   95.90%   +0.21%     
==========================================
  Files          69       69              
  Lines        4289     4123     -166     
==========================================
- Hits         4104     3954     -150     
+ Misses        185      169      -16     

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