perf: avoid extra calculation for hash update

[alexander-akait]

fixes #140

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 93.07%. Comparing base (dc41538) to head (16aae29).
Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/SourceMapSource.js	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #162      +/-   ##
==========================================
- Coverage   93.14%   93.07%   -0.07%     
==========================================
  Files          24       24              
  Lines        1779     1835      +56     
  Branches      566      596      +30     
==========================================
+ Hits         1657     1708      +51     
- Misses        112      118       +6     
+ Partials       10        9       -1     

Flag	Coverage Δ
integration	93.07% <87.50%> (-0.07%)	⬇️

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