osd: fix osd smart data collection segment fault issue.

osd: fix osd smart data collection segment fault issue.
output.read_fd() could return zero length. it needs to
be handled gracefully.

always call invoke join() if spawn() is successfully returned.
otherwise there will be assert failure in ~SubProcess().

Fixes: http://tracker.ceph.com/issues/23899
Signed-off-by: Gu Zhongyan <guzhongyan@360.cn>