Prevent infinite loop if 0 records are archived.

If _get_oldest_record() finds a record to archive/purge, then archive
should always archive at least one record. If for whatever reason it
fails to archive any records (_archive_table() returns a 0), then we
don't want call continue, but want to return an error. Calling continue
to go back to the beginning of the while loop would result in an
infinite loop.

Bug 6033.