update saturating calculation in rewards

[zjb0807]

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 65.40%. Comparing base (8285019) to head (d946f57).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
rewards/src/lib.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1016      +/-   ##
==========================================
- Coverage   65.43%   65.40%   -0.03%     
==========================================
  Files          49       49              
  Lines        3726     3726              
==========================================
- Hits         2438     2437       -1     
- Misses       1288     1289       +1     

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