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

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#### Checklist: <!-- Go over all points below, and after creating the PR, tick the checkboxes that apply --> - [x] PR should be backported to stable branches - [x] Tested changes locally - [x] Related to issue #3653

#### Description <!-- Describe your changes in detail -->

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. You can view, comment on, or merge this pull request online at:

https://github.com/kamailio/kamailio/pull/3697

-- Commit Summary --

* pv_core: Fix negative index bug for hfl()

-- File Changes --

M src/modules/pv/pv_core.c (6)

-- Patch Links --

https://github.com/kamailio/kamailio/pull/3697.patch https://github.com/kamailio/kamailio/pull/3697.diff