lua net.interfaces(): fix mac addresses

Only the first byte was being shown since 3ab77332.
I can't see at all why this part was changed;
the buffer is (and was) way overlong for this,
so writing one zero byte just after the end is OK.
27 jobs from net-interfaces-mac in 8 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
canceled #259468
docker linux amd64
archive

00:00:08

canceled #259469
docker linux amd64
build

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canceled #259470
docker linux amd64
build-asan

00:00:08

canceled #259471
docker
kres-gen

00:00:02

 
  Test
canceled #259472
docker delayed
build:darwin

canceled #259473
docker linux amd64
deckard

canceled #259474
docker delayed
doc

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dind
docker

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docker delayed
lint:other

canceled #259477
docker linux amd64 delayed
lint:pedantic

canceled #259478
docker linux amd64
lint:scan-build

canceled #259479
docker linux amd64
lint:tidy

canceled #259480
privileged
pkg:epel-7

canceled #259481
privileged
pkg:fedora-29

canceled #259482
docker linux amd64
respdiff:basic

canceled #259483
docker linux amd64 delayed
test:valgrind

 
  Extended
canceled #259485
respdiff
fwd-tls6-kresd.udp6

canceled #259486
respdiff
fwd-udp6-kresd.udp6

canceled #259490
respdiff
fwd-udp6-unbound.tcp6

canceled #259491
respdiff
fwd-udp6-unbound.tls6

canceled #259489
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fwd-udp6-unbound.udp6

canceled #259488
respdiff
iter.tls6

canceled #259487
respdiff
iter.udp6

canceled #259484
docker linux amd64
pytests

canceled #259492
respdiff
resperf:fwd-tls6.udp-asan

canceled #259493
respdiff
resperf:fwd-udp6.udp-asan

canceled #259494
respdiff
resperf:iter.udp-asan