rgw: prevent crash in `radosgw-admin bucket object shard ...`

rgw: prevent crash in `radosgw-admin bucket object shard ...`

This subcommand is used to ask radosgw-admin which bucket index shard
a given object in a given bucket would have its bucket index entry
on. The user is required to supply the number of shards (i.e., the
command doesn't look that up). If 0 is provided it would result in a
divide by zero runtime exception. Values less than or equal to zero
are now protected.

Signed-off-by: J. Eric Ivancich <ivancich@redhat.com>
(cherry picked from commit a2b76b0e09ece61fd0978411d08d13257fe6ddbe)