Running some statements that use IN subqueries outside context of a regular
query could result in server abnormal termination.

The reason for failure is that internal structures SELECT_LEX/SELECT_LEX_UNIT
created on behalf of parsed query were initialized incorrectly. Incorrect
initialization of the structures SELECT_LEX/SELECT_LEX_UNIT was introduced
by the commit de745ecf
(MDEV-11953: support of brackets in UNION/EXCEPT/INTERSECT operations)
pushed into 10.4, that is the reason this bug report is not reproduced in 10.3.

To fix the issue the method SLECTE_LEX::register_unit is used for proper
initialization of the data structures SELECT_LEX/SELECT_LEX_UNIT. Additionally,
the method SELECT_LEX::get_slave() was removed from the source code base
since for those use cases where it is used it can be replaced by the method
first_inner_unit().