bgpd: Announce labeled-unicast default-originate

bgpd: Announce labeled-unicast default-originate

Without this patch, this just crashes:

```
(gdb) bt
0  raise (sig=sig@entry=11) at ../sysdeps/unix/sysv/linux/raise.c:51
1  0x00007f66d977b10c in core_handler (signo=11, siginfo=0x7ffd87aa0430, context=<optimized out>) at lib/sigevent.c:261
2  <signal handler called>
3  stream_put_labeled_prefix (s=s@entry=0x55bd3ce53050, p=p@entry=0x7ffd87aa0a20, label=label@entry=0x0, addpath_capable=<optimized out>, addpath_tx_id=addpath_tx_id@entry=1)
    at lib/stream.c:1057
4  0x000055bd3bfba176 in bgp_packet_mpattr_prefix (s=s@entry=0x55bd3ce53050, afi=afi@entry=AFI_IP, safi=safi@entry=SAFI_LABELED_UNICAST, p=p@entry=0x7ffd87aa0a20, prd=prd@entry=0x0,
    label=label@entry=0x0, num_labels=0, addpath_capable=true, addpath_tx_id=1, attr=0x7ffd87aa2c20) at bgpd/bgp_attr.c:3964
5  0x000055bd3bfba2b5 in bgp_packet_attribute (bgp=0x55bd3cd8e470, bgp@entry=0x0, peer=peer@entry=0x55bd3cf21fc0, s=s@entry=0x55bd3ce53050, attr=attr@entry=0x7ffd87aa2c20,
    vecarr=vecarr@entry=0x7ffd87aa0a10, p=p@entry=0x7ffd87aa0a20, afi=AFI_IP, safi=SAFI_LABELED_UNICAST, from=0x7f66d9ba9010, prd=0x0, label=0x0, num_labels=0, addpath_capable=true,
    addpath_tx_id=1, bpi=0x0) at bgpd/bgp_attr.c:4139
6  0x000055bd3c04d455 in subgroup_default_update_packet (subgrp=subgrp@entry=0x55bd3cd885b0, attr=attr@entry=0x7ffd87aa2c20, from=from@entry=0x7f66d9ba9010) at bgpd/bgp_updgrp_packet.c:1129
7  0x000055bd3c04a9a5 in subgroup_default_originate (subgrp=0x55bd3cd885b0, withdraw=withdraw@entry=0) at bgpd/bgp_updgrp_adv.c:972
8  0x000055bd3c022668 in bgp_default_originate (peer=peer@entry=0x7f66d574a010, afi=afi@entry=AFI_IP, safi=safi@entry=SAFI_LABELED_UNICAST, withdraw=withdraw@entry=0)
    at bgpd/bgp_route.c:5037
9  0x000055bd3c0922e0 in peer_default_originate_set (peer=0x7f66d574a010, afi=afi@entry=AFI_IP, safi=safi@entry=SAFI_LABELED_UNICAST, rmap=rmap@entry=0x0, route_map=route_map@entry=0x0)
    at bgpd/bgpd.c:5428
10 0x000055bd3c076c07 in peer_default_originate_set_vty (set=1, rmap=0x0, safi=SAFI_LABELED_UNICAST, afi=AFI_IP, peer_str=<optimized out>, vty=0x55bd3ce4c900) at bgpd/bgp_vty.c:6941
11 neighbor_default_originate (self=<optimized out>, vty=0x55bd3ce4c900, argc=<optimized out>, argv=<optimized out>) at bgpd/bgp_vty.c:6958
12 0x00007f66d9721dc0 in cmd_execute_command_real (vline=vline@entry=0x55bd3cd874d0, vty=vty@entry=0x55bd3ce4c900, cmd=cmd@entry=0x0, up_level=up_level@entry=0, filter=FILTER_RELAXED)
    at lib/command.c:996
13 0x00007f66d9721f39 in cmd_execute_command (vline=vline@entry=0x55bd3cd874d0, vty=vty@entry=0x55bd3ce4c900, cmd=cmd@entry=0x0, vtysh=vtysh@entry=0) at lib/command.c:1055
14 0x00007f66d9722162 in cmd_execute (vty=vty@entry=0x55bd3ce4c900, cmd=cmd@entry=0x55bd3cd8a230 "neighbor 192.168.34.4 default-originate ", matched=matched@entry=0x0, vtysh=vtysh@entry=0)
    at lib/command.c:1223
15 0x00007f66d9792337 in vty_command (vty=vty@entry=0x55bd3ce4c900, buf=<optimized out>) at lib/vty.c:486
16 0x00007f66d9792570 in vty_execute (vty=0x55bd3ce4c900) at lib/vty.c:1249
```

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>