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.