This bug affected some queries with an IN/ALL/ANY predicand or an EXISTS
predicate whose subquery contained a GROUP BY clause that could be
eliminated. If this clause used a IN/ALL/ANY predicand whose left operand
was a single-value subquery then execution of the query caused a crash of
the server after invokation of remove_redundant_subquery_clauses().
The crash was caused by an attempt to exclude the unit for the single-value
subquery from the query tree for the second time by the function
Item_subselect::eliminate_subselect_processor().

This bug had been masked by the bug MDEV-28617 until a fix for the latter
that properly excluded units was pushed into 10.3.

Approved by Oleksandr Byelkin <sanja@mariadb.com>