rgw: fix uninit ofs in RGWObjManifect::obj_iterator

rgw: fix uninit ofs in RGWObjManifect::obj_iterator

Valgrind picked this up:

  <kind>UninitCondition</kind>
  <what>Conditional jump or move depends on uninitialised value(s)</what>
  <stack>
    <frame>
      <ip>0x5145B8</ip>
      <obj>/usr/bin/radosgw</obj>
      <fn>RGWObjManifest::obj_iterator::seek(unsigned long)</fn>
      <dir>/srv/autobuild-ceph/gitbuilder.git/build/out~/ceph-0.82-354-g62027ec/src/rgw</dir>
      <file>rgw_rados.cc</file>
      <line>562</line>
    </frame>
    <frame>
      <ip>0x5672A4</ip>
      <obj>/usr/bin/radosgw</obj>
      <fn>list_multipart_parts(RGWRados*, req_state*, std::string const&amp;, std::string&amp;, int, int, std::map&lt;unsigned int, RGWUploadPartInfo, std::less&lt;unsigned int&gt;, std::allocator&lt;std::pair&lt;unsigned int const, RGWUploadPartInfo&gt; &gt; &gt;&amp;, int*, bool*, bool)</fn>
      <dir>/srv/autobuild-ceph/gitbuilder.git/build/out~/ceph-0.82-354-g62027ec/src/rgw</dir>
      <file>rgw_rados.h</file>
      <line>217</line>
    </frame>
    <frame>
      <ip>0x5688EE</ip>
      <obj>/usr/bin/radosgw</obj>
      <fn>RGWListMultipart::execute()</fn>
      <dir>/srv/autobuild-ceph/gitbuilder.git/build/out~/ceph-0.82-354-g62027ec/src/rgw</dir>
      <file>rgw_op.cc</file>
      <line>2956</line>
    </frame>
...

Fixes: #8699
Backport: firefly
Signed-off-by: Sage Weil <sage@inktank.com>