Fixed "zero size buf in output" alerts.

If a request had an empty request body (with Content-Length: 0), and there
were preread data available (e.g., due to a pipelined request in the buffer),
the "zero size buf in output" alert might be logged while proxying the
request to an upstream.

Similar alerts appeared with client_body_in_file_only if a request had an
empty request body.