Do not log error:transaction-end-before-headers after invalid requests.

Do not log error:transaction-end-before-headers after invalid requests.

Squid was not consuming read leftovers after failing to parse a request.
Starting with r14752, those leftovers were misinterpreted as another
unparsed request, creating an extra error:transaction-end-before-headers
access.log line after every error:invalid-request line (and probably
after every error:request-too-large line).

To stop Squid from accidentally reading new bytes and misinterpreting
them as another request, I was tempted to also clear flags.readMore
after consuming unparsable leftovers. In my tests, the flag is cleared
in ConnStateData::quitAfterError() called from clientTunnelOnError(),
but that logic looks rather fragile. I resisted the temptation to
improve it because controlling reads is a complicated matter (especially
in on_unsupported_protocol context) outside this logging fix scope.