lib: Fix memory leak in `zclient_send_localsid()`

lib: Fix memory leak in `zclient_send_localsid()`

Running `bgp_srv6l3vpn_to_bgp_vrf` and `bgp_srv6l3vpn_to_bgp_vrf2`
topotests with `--valgrind-memleaks` gives several memory leak errors.
This is due to the way FRR daemons pass local SIDs to zebra: to send a
local SID to zebra, FRR daemons call the `zclient_send_localsid()`
function.

The `zclient_send_localsid()` function performs the following sequence
of operations:
* create a temporary `struct nexthop`;
* call `nexthop_add_srv6_seg6local()` to fill the `struct nexthop` with
the proper local SID information;
* create a `struct zapi_route` and call `zapi_nexthop_from_nexthop()` to
copy the information from the `struct nexthop` to the
`struct zapi_route`;
* send the `struct zapi_route` to zebra through the ZAPI.

The `nexthop_add_srv6_seg6local()` function uses `XCALLOC()` to allocate
memory for the SRv6 nexthop. This memory is never freed.

Creating a temporary `struct nexthop` is unnecessary, as the local SID
information can be pushed directly to the `struct zapi_route`. This
patch simplifies the implementation of `zclient_send_localsid()` by
avoiding using the temporary `struct nexthop`. This eliminates the need
to use `nexthop_add_srv6_seg6local()` to fill the `struct nexthop` and
consequently fixes the memory leak.

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