There is no way to resolve situations when mysql_real_connect_cont (after called repeatedly until 0 is returned) fails to initialize a connection:

If one tries to call mysql_close (or async counterpart) on the mysql handler, mysql_close invokes use after free or double free.
If one doesn't call mysql_close, then the memory is leaked (probably options, but not sure https://github.com/mariadb-corporation/mariadb-connector-c/blob/3.3/libmariadb/mariadb_lib.c#L2011).

Use after free comes from here:
first free: https://github.com/mariadb-corporation/mariadb-connector-c/blob/3.3/libmariadb/mariadb_lib.c#L1729
second free: https://github.com/mariadb-corporation/mariadb-connector-c/blob/3.3/libmariadb/mariadb_lib.c#L2382

If this is indeed a bug this renders mariadb-connector-c async functionality borderline unusable or at the very least unsound in long-running server applications.

Repro is attached, one could either follow the README to run it dockerized or just test on host machine