fix: validate optional/extension header bytes are present

[gruyaume]

Description

Validate that the optional/extension header bytes are actually present before the parser advances past them, and fail closed (drop the packet cleanly) if they aren't.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that validate the behaviour of the software
• I validated that new and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules