Fix IAMF specific box parsing bug in MP4 demuxer

[osagie98]

The IamfSpecificBox::Parse method previously sliced the buffer from the
start of the box header instead of the current reader position. This
resulted in the IAMF configuration OBUs being parsed incorrectly
because the offset from previous reads within the box was not being
respected.

Issue: 537956110

[gemini-code-assist]

Code Review

This pull request updates the buffer offset in IamfSpecificBox::Parse within media/formats/mp4/box_definitions.cc to use the current reader position. However, a critical security vulnerability was identified: the code does not validate whether the reader has enough remaining bytes before creating the span and copying its contents. This could lead to an out-of-bounds read if config_obus_size is larger than the remaining buffer size. It is recommended to add a validation check using reader->HasBytes(config_obus_size) before accessing the buffer.

[github-actions]

🤖 Gemini Suggested Commit Message

---

media: Fix IAMF box parsing in MP4 demuxer

The IamfSpecificBox::Parse method previously sliced the buffer from the
start of the box header instead of the current reader position. This
resulted in the IAMF configuration OBUs being parsed incorrectly
because the offset from previous reads within the box was not being
respected.

Issue: 537956110

---

💡 Pro Tips for a Better Commit Message:

1. Influence the Result: Want to change the output? You can write custom prompts or instructions directly in the Pull Request description. The model uses that text to generate the message.
2. Re-run the Generator: Post a comment with: /generate-commit-message

[borongc]

RS approval, and waiting for our partners to verify it.