The select with idbExtentMax in projection returns an error.

MariaDB [test]> select idbExtentMax(i) from c1;

ERROR 1815 (HY000): Internal error: Error while fetching from ExeMgr: IDB-2035: An internal error occurred.  Check the error log file & contact support.

According with the call stack plugin code incorrectly treats the MDB field. MDB sends down Field_string but MCS plugin code treats it like an integer. This forces code to throw.
A side note. TypeHandler code uses dynamic_cast for asserts that is a heavyweight reflection op. It must be replaced with typeid.