Test: fixing a compile warning in ceph_objectstore_tool.cc

For the compiler's sake:
tools/ceph_objectstore_tool.cc:2547:15: warning: ‘r’ may be used
uninitialized in this function [-Wmaybe-uninitialized]

Signed-off-by: Zhiqiang Wang <wonzhq@hotmail.com>
Merge pull request #2353 from wonzhq/compile-warn

Test: fixing a compile warning in ceph_objectstore_tool.cc

Reviewed-by: Sage Weil <sage@redhat.com>