daemon TCP to upstream: don't send wrong message length

See the added comments.  Such bugs are tricky, because the old code
would typically work just fine, only if libuv/OS decided to postpone
copying the data (perhaps large load), we would send two bytes from
this address on C stack - their later value (hard to predict what).

Security risks: the two bytes might theoretically contain information
that was more or less private and we just send it to some DNS server
(possibly over unencrypted TCP), but ATM I find it very unlikely that
this bug could be practically exploited.
27 jobs for <span class="ref-name">p/uv_write-lifetime</span> in 27 minutes and 52 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Build
passed #268540
docker linux amd64
archive

00:00:55

passed #268541
docker linux amd64
build

00:01:02

passed #268542
docker linux amd64
build-asan

00:01:04

passed #268543
docker
kres-gen

00:00:59

 
  Test
passed #268544
docker delayed
build:darwin

00:00:22

passed #268545
docker linux amd64
deckard

00:02:34

passed #268546
docker delayed
doc

00:01:12

passed #268547
dind
docker

00:02:14

passed #268548
docker delayed
lint:other

00:00:52

passed #268549
docker linux amd64 delayed
lint:pedantic

00:01:13

passed #268550
docker linux amd64
lint:scan-build

00:02:23

passed #268551
docker linux amd64
lint:tidy

00:03:00

passed #268552
privileged
pkg:epel-7

00:02:05

passed #268553
privileged
pkg:fedora-29

00:02:39

passed #268554
docker linux amd64
respdiff:basic

00:04:49

passed #268555
docker linux amd64 delayed
test:valgrind

00:02:13

 
  Extended
failed #268557
respdiff
fwd-tls6-kresd.udp6

00:11:20

passed #268558
respdiff
fwd-udp6-kresd.udp6

00:09:53

passed #268562
respdiff
fwd-udp6-unbound.tcp6

00:15:23

passed #268563
respdiff
fwd-udp6-unbound.tls6

00:19:53

passed #268561
respdiff
fwd-udp6-unbound.udp6

00:21:53

passed #268560
respdiff
iter.tls6

00:19:54

passed #268559
respdiff
iter.udp6

00:10:52

passed #268556
docker linux amd64
pytests

00:04:21

passed #268564
respdiff
resperf:fwd-tls6.udp-asan

00:16:18

passed #268565
respdiff
resperf:fwd-udp6.udp-asan

00:12:48

passed #268566
respdiff
resperf:iter.udp-asan

00:08:17

 
Name Stage Failure
failed
fwd-tls6-kresd.udp6 Extended
Running after script...
$ cp -t . respdiff_commitdir/$RESPDIFF_TEST/j* ||:
$ cp -t . respdiff_commitdir/*$RESPDIFF_TEST*.png ||:
Uploading artifacts...
Runtime platform arch=amd64 os=linux pid=124203 revision=5a147c92 version=11.11.1
./j*: found 11 matching files
./*.png: found 3 matching files
Uploading artifacts to coordinator... ok id=268557 responseStatus=201 Created token=KqQFDhri
ERROR: Job failed: exit status 1