Fix incorrect colon indicator scanning

[semihbkgr]

fixes #484

isNextEOS should return true when the next idx is out of src

• Describe the purpose for which you created this PR.
• Create test code that corresponds to the modification

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.76%. Comparing base (558921a) to head (bd078f0).
Report is 2 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #485      +/-   ##
==========================================
- Coverage   77.85%   77.76%   -0.09%     
==========================================
  Files          17       17              
  Lines        5797     5819      +22     
==========================================
+ Hits         4513     4525      +12     
- Misses       1013     1022       +9     
- Partials      271      272       +1     

[goccy]

Please add the test case for fixing

[goccy]

Thank you for your contribution ! LGTM 👍