This is the MariaDB equivalent of fixing the MySQL 5.7 regression
Bug #26935001 ALTER TABLE AUTO_INCREMENT TRIES TO READ
INDEX FROM DISCARDED TABLESPACE

Oracle did not publish a test case, but it is easy to guess
based on the commit message. The MariaDB code is different
due to MDEV-6076 implementing persistent AUTO_INCREMENT.

commit_set_autoinc(): Report ER_TABLESPACE_DISCARDED if the
tablespace is missing.

prepare_inplace_alter_table_dict(): Avoid accessing a discarded
tablespace. (This avoids generating warnings in fil_space_acquire().)