This bug could manifest itself after pushing a where condition over a
mergeable derived table / view / CTE DT into a grouping view / derived
table / CTE V whose item list contained set functions with constant
arguments such as MIN(2), SUM(1) etc. In such cases the field references
used in the condition pushed into the view V that correspond set functions
are wrapped into Item_direct_view_ref wrappers. Due to a wrong implementation
of the virtual method const_item() for the class Item_direct_view_ref the
wrapped set functions with constant arguments could be erroneously taken
for constant items. This could lead to a wrong result set returned by the
main select query in 10.2. In 10.4 where a possibility of pushing condition
from HAVING into WHERE had been added this could cause a crash.

Approved by Sergey Petrunya <sergey.petrunya@mariadb.com>