bgpd: fix labels static-analyser (backport #16655)

[mergify]

Fix static-analyser warnings with BGP labels:

$ scan-build make -j12
bgpd/bgp_updgrp_packet.c:819:10: warning: Access to field 'extra' results in a dereference of a null pointer (loaded from variable 'path') [core.NullDereference]
? &path->extra->labels->label[0]
^~~~~~~~~

---

This is an automatic backport of pull request #16655 done by Mergify.

[mergify]

Cherry-pick of a152692 has failed:

On branch mergify/bp/stable/10.1/pr-16655
Your branch is up to date with 'origin/stable/10.1'.

You are currently cherry-picking commit a152692f5a.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   bgpd/bgp_bmp.c
	modified:   bgpd/bgp_evpn.c
	modified:   bgpd/bgp_label.c
	modified:   bgpd/bgp_mac.c
	modified:   bgpd/bgp_mplsvpn.c
	modified:   bgpd/bgp_route.c
	modified:   bgpd/bgp_route.h
	modified:   bgpd/bgp_routemap.c
	modified:   bgpd/bgp_rpki.c
	modified:   bgpd/bgp_updgrp_packet.c
	modified:   bgpd/bgp_zebra.c
	modified:   bgpd/rfapi/rfapi_import.c
	modified:   bgpd/rfapi/rfapi_rib.c
	modified:   bgpd/rfapi/rfapi_vty.c
	modified:   bgpd/rfapi/vnc_import_bgp.c

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   bgpd/bgp_nht.c

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[ton31337]

Created a manual backport #16671.