Problem: We created more than 5 encryption keys for redo-logs.
Idea was that we do not anymore create more than one encryption
key for redo-logs but if existing checkpoint from earlier
MariaDB contains more keys, we should read all of them.

Fix: Add new encryption key to memory structure only if there
currently has none or if we are reading checkpoint from the log.
Checkpoint from older MariaDB version could contain more than
one key.