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

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.