rgw: fix `radosgw-admin object unlink ...`

rgw: fix `radosgw-admin object unlink ...`

The unlink subcommand did not handle unsharded bucket indices
appropriately. These are when the number of shards listed in the
bucket instance object is 0. In that case there will actually be 1
shard.

When number of shards as 0 is passed into the function that maps
object names to shards, it returns -1. And that was not handled
properly. That is now fixed.

Signed-off-by: J. Eric Ivancich <ivancich@redhat.com>
(cherry picked from commit 9eeb71e28526df8ce2b920d4f50763a734264416)