Fix G115 false positives for guarded int64-to-byte conversions

[ccojocar]

fixes #1577

[ravisastryk]

LGTM

[codecov]

Codecov Report

❌ Patch coverage is 71.73913% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.71%. Comparing base (3c9c3da) to head (aa00828).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
analyzers/conversion_overflow.go	71.73%	8 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1578      +/-   ##
==========================================
- Coverage   80.73%   80.71%   -0.03%     
==========================================
  Files         104      104              
  Lines        9852     9878      +26     
==========================================
+ Hits         7954     7973      +19     
- Misses       1412     1418       +6     
- Partials      486      487       +1     

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[ldemailly]

@ccojocar thanks for the quick fix, yet it doesn't seem to fix it unless I did something wrong #1577 (comment)