bgpd: fix allocating uninitialized bgp_labels

bgpd: fix allocating uninitialized bgp_labels

Memory was not zeroed at allocation.

> ==2604268== Conditional jump or move depends on uninitialised value(s)
> ==2604268==    at 0x26F026: bgp_labels_unintern (bgp_label.c:116)
> ==2604268==    by 0x30000C: adj_free (bgp_updgrp_adv.c:81)
> ==2604268==    by 0x3019EE: bgp_adj_out_remove_subgroup (bgp_updgrp_adv.c:728)
> ==2604268==    by 0x30537C: subgroup_withdraw_packet (bgp_updgrp_packet.c:1042)
> ==2604268==    by 0x2970AF: bgp_generate_updgrp_packets (bgp_packet.c:508)
> ==2604268==    by 0x49E06F1: event_call (event.c:2001)
> ==2604268==    by 0x495AB8B: frr_run (libfrr.c:1238)
> ==2604268==    by 0x200C4B: main (bgp_main.c:555)

Fixes: 3c86f776f0 ("bgpd: add bgp_labels hash")
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>
(cherry picked from commit dd10379b3e4c544874306fbb5efec5afc1cf1f9e)