fix(context): enhance code imported by #3413

[zjj]

if it needs to check if the handler is nil, tie c.index shall always ++

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[appleboy]

@zjj Please help to write testing.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.99%. Comparing base (3dc1cd6) to head (90815bf).
Report is 79 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4083      +/-   ##
==========================================
- Coverage   99.21%   98.99%   -0.22%     
==========================================
  Files          42       44       +2     
  Lines        3182     3387     +205     
==========================================
+ Hits         3157     3353     +196     
- Misses         17       24       +7     
- Partials        8       10       +2     

Flag	Coverage Δ
	?
-tags "sonic avx"	98.98% <100.00%> (?)
-tags go_json	98.98% <100.00%> (?)
-tags nomsgpack	98.98% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	98.99% <100.00%> (-0.22%)	⬇️
go-1.22	98.99% <100.00%> (?)
macos-latest	98.99% <100.00%> (-0.22%)	⬇️
ubuntu-latest	98.99% <100.00%> (-0.22%)	⬇️

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[zjj]

@zjj Please help to write testing.

I would love to write some tests, but with my limited knowledge, I really can't think of how to test this here. ...