testing/fatutf8:fix Out of bounds writing

[xiaoxiang781216]

Summary

fix coverity bug

Impact

minor fix

Testing

ci

[nuttxpr]

[Experimental Bot, please feedback here]

No, this PR does not meet the NuttX requirements.

Missing Information:

• Summary:
  • Lacks a clear explanation of the bug and the solution. What specifically was the Coverity issue (e.g., resource leak, potential null pointer dereference)? How was it fixed? Which part of the codebase was affected?
• Impact:
  • "Minor fix" is too vague. While it might be minor, specify potential areas of impact. Does this fix affect any specific architectures, drivers, or configurations? Even if the answer is no to most impact questions, state that explicitly.
• Testing:
  • "ci" is insufficient. Provide:
    • Specific build hosts used (OS, CPU architecture, compiler version).
    • Specific target platforms tested (architecture, board, configuration).
    • Relevant snippets of testing logs (ideally showing the issue before and the resolution after the fix).

Example of an Improved Summary:

This PR resolves a Coverity-flagged resource leak in the I2C driver for the STM32F4 family. The issue occurred when an I2C transfer was aborted prematurely, failing to release a mutex. The fix ensures the mutex is always released, even in error conditions.