Fix UAF bug in read_ihex_chunks() when first chunk is reallocated

[SiteRelEnby]

Commit e473c83 partially fixed UAF errors by introducing backlink_p to update the previous chunk's next pointer after realloc. However, when the first chunk is reallocated, backlink_p is NULL, so the root pointer (*chunks_p) never gets updated, leaving it pointing to freed memory.

This causes a use-after-free/double-free when read_ihex_file() later tries to free the chunk list.

Fix by updating *chunks_p directly when backlink_p is NULL (first chunk realloc case).

Valgrind confirmed this eliminates the UAF errors.

[gatk555]

Thanks. It looks good, but the new variable seems unnecessary. Why not just "*chunks_p = chunk"? Also, please can you reduce the check-in comment to one line, and move the original to "Conversation". It is a good explanation, but the history does not need so much detail, and I would prefer to merge without modifying.