librados: fix ObjectIterator::operator= for the end iterator

We can't set a shared_ptr to NULL, we need to reset it instead. Add
another test for various permutations of this.

Fixes: #7538
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Merge pull request #1322 from ceph/wip-librados-end-iterator

librados: fix ObjectIterator::operator= for the end iterator

Reviewed-by: Sage Weil <sage@inktank.com>