sheep: mute gcc warning of uninitialized devnull_fd

sheep.c:446:8: warning: ‘devnull_fd’ may be used uninitialized in this function [-Wuninitialized]
sheep.c:395:11: note: ‘devnull_fd’ was declared here

Just init devnull_fd to mute the warning.

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>