In commit 685d958e (MDEV-14425)
a bug was introduced to mariadb-backup --backup for the case when
the log is wrapping around to log_sys.START_OFFSET (12288).

This could also cause a "Missing FILE_CHECKPOINT" error during
mariadb-backup --prepare, in case the log copying resumed after
the server had produced a multiple of innodb_log_file_size-12288
bytes of more log so that the last mini-transaction would end
exactly at the end of the log file.

xtrabackup_copy_logfile(): If the log wraps around, read everything
to the end of the log file, and then the rest from log_sys.START_OFFSET.