ndp_table: For unspecified address, return broadcast ethernet address

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.