rgw: `radosgw-admin bucket stats` on indexless bucket crashes

rgw: `radosgw-admin bucket stats` on indexless bucket crashes

The new bucket layout code didn't check whether the bucket is
indexless prior to asking for the last entry in the layout log. The
layout log appears to be empty for an indexless bucket, thereby
putting the runtime in an undefined state that later may cause a
failed assertion.

This commit adds two safety checks and returns -EINVAL along with
putting useful information on stderr when either stats are requested
on an indexless bucket or when the layout log is empty.

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