rgw: fix FP error when calculating enteries per bi shard

rgw: fix FP error when calculating enteries per bi shard

When calculating how many entries per shard to request during an
ordered bucket listing, we divide by the number of bucket index
shards. If this value is 0, then a floating point exception is
generated, crashing the RGW.

This addresses the proximate issue by detecting the situation and
returning an error rather than crashing.

Signed-off-by: J. Eric Ivancich <ivancich@redhat.com>