rados_bencher: re-add written objects constraint to read benchmark.
rados_bencher: re-add written objects constraint to read benchmark.

Somehow, in the last major change, the constraints that kept the
bencher from trying to read non-existent objects got removed. Put
a check back in the main bench loop to fix that.

Signed-off-by: Greg Farnum <0bcb4295d38ee745130ea5f2bb88ef4eef474e61@dreamhost.com>

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph