lib, bgpd: Enhance `srv6_locator_chunk_free()` API

lib, bgpd: Enhance `srv6_locator_chunk_free()` API

A programmer can use the `srv6_locator_chunk_free()` function to free
the memory allocated for a `struct srv6_locator_chunk`.

The programmer invokes `srv6_locator_chunk_free()` by passing a single
pointer to the `struct srv6_locator_chunk` to be freed.
`srv6_locator_chunk_free()` uses `XFREE()` to free the memory.
It is the responsibility of the programmer to set the
`struct srv6_locator_chunk` pointer to NULL after freeing memory with
`srv6_locator_chunk_free()`.

This commit modifies the `srv6_locator_chunk_free()` function to take a
double pointer instead of a single pointer. In this way, setting the
`struct srv6_locator_chunk` pointer to NULL is no longer the
programmer's responsibility but is the responsibility of
`srv6_locator_chunk_free()`. This prevents programmers from making
mistakes such as forgetting to set the pointer to NULL after invoking
`srv6_locator_chunk_free()`.

Signed-off-by: Carmine Scarpitta <carmine.scarpitta@uniroma2.it>