RTEMS | dosfs: Avoid FAT file cache update after failed write (!1346)

[Yang Zhang (@Chloe)]

Yang Zhang created a merge request: https://gitlab.rtems.org/rtems/rtos/rtems/-/merge_requests/1346

Project:Branches: Chloe/rtems:fix-dosfs-write-cache to rtems/rtos/rtems:main
Author:   Yang Zhang



## Summary

In fat_file_write_fat32_or_non_root_dir(), the FAT file cache was updated even if the first fat_cluster_write() failed and no bytes were completed.

\
In that case, cmpltd remains zero, so the expression using cmpltd - 1 may underflow and the cache may be updated with an invalid file cluster or disk cluster mapping.

Only update the cache if at least one byte was successfully written.

## Generative AI

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View it on GitLab: https://gitlab.rtems.org/rtems/rtos/rtems/-/merge_requests/1346
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