If a query has a HAVING clause that contains a predicate with a constant
IN subquery whose lef part in its turn is a subquery and the predicate is
subject to pushdown from HAVING to WHERE then execution of the query could
cause a crash of the server.

The cause of the problem was the missing implementation of the walk()
method for the class Item_in_optimizer. As a result in some cases the left
operand of the Item_in_optimizer condition could be traversed twice by
the walk procedure. For many call-back functions used as an argument of
this procedure it does not matter. Yet it matters for the call-back
function cleanup_excluding_immutables_processor() used in pushdown of
predicates from HAVING to WHERE. If the processed item is marked with
the IMMUTABLE_FL flag then the processor just removes this flag, otherwise
it performs cleanup of the item making it unfixed. If an item is marked
with an the IMMUTABLE_FL and it traversed with this processor twice then
it becomes unfixed after the second traversal though the flag indicates
that the item should not be cleaned up.

Approved by Oleksandr Byelkin <sanja@mariadb.com>