We cannot let the guest crash libslirp by making it answer a tftp
request such as shown in
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=33873
https://gitlab.com/qemu-project/qemu/-/issues/111

unspecified addresses may also be used for non-configured devices, so it
makes sense to use the broadcast ethernet address in that case, just
like we do with IPv4.