pkt-line: check write_in_full() errors against "< 0"

pkt-line: check write_in_full() errors against "< 0"

As with the previous two commits, we prefer to check
write_in_full()'s return value to see if it is negative,
rather than comparing it to the input length.

These cases actually flip the logic to check for success,
making conversion a little different than in other cases. We
could of course write:

  if (write_in_full(...) >= 0)
          return 0;
  return error(...);

But our usual method of spelling write() error checks is
just "< 0". So let's flip the logic for each of these
conditionals to our usual style.

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>