test: add zero boundary assertions to TestSafeInt8 and TestSafeUint16

[aoright]

Summary

• In utils_test.go, add 0 value boundary checks to TestSafeInt8 and TestSafeUint16.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.39%. Comparing base (3dc1cd6) to head (d075bf0).
⚠️ Report is 292 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4810      +/-   ##
==========================================
- Coverage   99.21%   98.39%   -0.83%     
==========================================
  Files          42       48       +6     
  Lines        3182     3172      -10     
==========================================
- Hits         3157     3121      -36     
- Misses         17       41      +24     
- Partials        8       10       +2     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.37% <ø> (?)
-tags go_json	98.25% <ø> (?)
-tags nomsgpack	98.30% <ø> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.25	98.32% <ø> (?)
go-1.26	98.39% <ø> (?)
macos-latest	98.39% <ø> (-0.83%)	⬇️
ubuntu-latest	98.32% <ø> (-0.89%)	⬇️

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