rc/test/osdc/object_cacher_stress.cc: fix comparison

rc/test/osdc/object_cacher_stress.cc: fix comparison

Clang complains:
  /home/jenkins/workspace/ceph-freebsd/src/test/osdc/object_cacher_stress.cc:50:26: error: ordered comparison between pointer and zero ('std::atomic<unsigned int> *' and 'int')
      assert(m_outstanding > 0);
               ~~~~~~~~~~~~~ ^ ~
  /home/jenkins/workspace/ceph-freebsd/src/include/assert.h:117:5: note: expanded from macro 'assert'
    ((expr)                                                               \
        ^~~~
Signed-off-by: Willem Jan Withagen <wjw@digiware.nl>