[v2.12]fix: handle NULL json_str case to avoid undefined behavior

[ZhouBox]

No description provided.

[Copilot]

Pull Request Overview

This PR addresses a potential undefined behavior issue in the MQTT plugin by adding a NULL check for the json_str variable before using it in string operations. The fix prevents crashes or undefined behavior that could occur when the JSON string generation fails or returns NULL.

• Adds NULL pointer validation for json_str before calling strlen()
• Wraps the string length calculation in a conditional block to ensure safe execution

[Copilot]

The closing brace appears to be misaligned or creates unbalanced braces. The NULL check for json_str should wrap only the strlen() operation, but this closing brace suggests it may be closing an outer scope incorrectly.

Suggested change

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.66%. Comparing base (9cca71b) to head (f7abe05).

Additional details and impacted files

@@            Coverage Diff             @@
##            v2.12    #2522      +/-   ##
==========================================
- Coverage   66.71%   66.66%   -0.05%     
==========================================
  Files         121      121              
  Lines       27550    27551       +1     
==========================================
- Hits        18379    18368      -11     
- Misses       9171     9183      +12     

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