teuthology/orchestra/daemon: Check if stdin is valid
teuthology/orchestra/daemon: Check if stdin is valid

As there is self.proc.stdin.close() in stop(), there are chances
of write failing with IOError("File is closed").

Signed-off-by: Jos Collin <95c525d980df66625968c3cbc156a20b1f05d67c@redhat.com>

ktdreyer/teuthology,ktdreyer/teuthology,ceph/teuthology,dmick/teuthology,SUSE/teuthology,ceph/teuthology,SUSE/teuthology,dmick/teuthology,dmick/teuthology,SUSE/teuthology