common/obj_bencher.cc: fix verification crashing when there's no objects

common/obj_bencher.cc: fix verification crashing when there's no objects

When specified pool is empty or doesn't contain valid bench objects, rados
can crash because it attempts to memcmp through NULL pointer. Added check
for valid object size (which also fixes the case when objects are there, but
they're truncated), and made object_size being size_t instead of int.
In seq_read_bench, there's a missing lock.Lock() which causes rados to trigger
assert when object reading fails. Moved lock.Lock() before completion_ret()
call.

Signed-off-by: Piotr Dałek <piotr.dalek@ts.fujitsu.com>