Backing up table that "doesn't exist in engine" cause crash in mariabackup when using encryption

When there is file for a table in a database directory in the datadir which is not in the data dictionary for InnoDB and tables are encrypted, then running mariaback will crash mariabackup with a core dump.
How to reproduce:
Install MariaDB 10.1.26 and MariaDB-Backup
$ sudo bash

1. service mysql start
2. mysql -u root test -e "CREATE TABLE t1(c1 INT)"
3. mysql -u root test -e "INSERT INTO t1 VALUES(1)"
4. service mysql stop
5. cd /var/lib/mysql/test
6. cp t1.frm t1_1.frm
7. cp t1.ibd t1_1.ibd
8. chown mysql:mysql t1_1.*
9. service mysql start
10. mkdir /tmp/backup
11. mariabackup --backup --target-dir=/tmp/backup/ --user=root
    Crash!