mariadb: bump to 10.1.38

mariadb: bump to 10.1.38

Upstream Release Notes:

- MDEV-17475: Maximum value of table_definition_cache is now 2097152
- MDEV-13671: InnoDB should use case-insensitive column name comparisons
  like the rest of the server
- ALTER TABLE fixes: MDEV-17230, MDEV-16499, MDEV-17904, MDEV-17833,
  MDEV-17470, MDEV-18237, MDEV-18016
- Improvements to InnoDB page checksum, recovery, and Mariabackup:
  MDEV-17957, MDEV-12112, MDEV-18025, MDEV-18279, MDEV-18183
- Galera
  - MDEV-15740: Galera durability fix
  - New configuration variable wsrep_certification_rules, used for
    controlling whether to use new/optimized
    (--wsrep_certification_rules=optimized) certification rules or the
    old/classic ones (--wsrep_certification_rules=strict). Setting the
    variable to strict can cause more certification failures.

- Fixes for the following security vulnerabilities:
  - CVE-2019-2537
  - CVE-2019-2529

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>