pv_core: Fix negative index bug for hfl()

[xkaraman]

Before: -1 yielded null and -2 the last element of a header

Pre-Submission Checklist

• Commit message has the format required by CONTRIBUTING guide
• Commits are split per component (core, individual modules, libs, utils, ...)
• Each component has a single commit (if not, squash them into one commit)
• No commits to README files for modules (changes must be done to docbook files
  in doc/ subfolder, the README file is autogenerated)

Type Of Change

• Small bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds new functionality)
• Breaking change (fix or feature that would change existing functionality)

Checklist:

• PR should be backported to stable branches
• Tested changes locally
• Related to issue pv: hfl(name)[N] for negative index bug #3653

Description

This PR fixes a bug referenced in #3653.

Before fix $(hfl(HEADER)[-1] where HEADER='Via' | Contact | Route | Record-Route produced null instead of the last element of header.

[miconda]

Thanks!

I thought it was based on the same approach as for $(hdr(name)[index]), but that works fine.