netlink: do not memcpy 0 bytes from a NULL pointer

[emaste]

Summary

Although it works in practice it is UB, and might break after a future
compiler update.

Sponsored by: The FreeBSD Foundation

[emaste]

sys/netlink/route/iface.c

464

free(NULL) is fine of course, but I think including it in the if body is more clear

[emaste]

Just missed 055776c84a41bb88c6ccd67615ea172864f6804c :)

[emaste]

Although that said I think we ought to include either D38927 or 055776c84a41bb88c6ccd67615ea172864f6804c in 13.2.