rgw: bucket index list produces incorrect result when non-ascii entries

rgw: bucket index list produces incorrect result when non-ascii entries

A recent PR that helped address the issue of non-ascii plain entries
didn't cover all the bases, allowing I/O errors to be produced in some
circumstances during a bucket index list (i.e., `radosgw-admin bi list
...`).

This fixes those issue and does some additional clean-up.

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