sheep: don't print error message when connection is off

...
Dec 17 19:04:27  ERROR [net 11088] do_read(220) connection is closed (48 bytes left)
Dec 17 19:04:27  ERROR [net 11088] rx_work(684) failed to read a header
...

This is quit annoying since nothing wrong with it and sheep.log is flooded with
these messages.

Signed-off-by: Liu Yuan <namei.unix@gmail.com>