The problem happened when running mariabackup agains a pre-MDEV-30971 server,
i.e. not having yet the system variable @@aria_log_dir_path.

As a result, backup_start() called the function backup_files_from_datadir()
with a NULL value, which further caused a crash.

Fix:
Perform this call:

    backup_files_from_datadir(.., aria_log_dir_path, ..)

only if aria_log_dir_path is not NULL. Otherwise,
assume that Aria log files are in their default location,
so they've just copied by the previous call:

    backup_files_from_datadir(.., fil_path_to_mysql_datadir, ..)

Thanks to Walter Doekes for a patch proposal.