As a result of this bug the second execution of the prepared statement
created for select from materialized view could return a wrong result set if
- the specification of the view used a left join
- an inner table the left join was a mergeable derived table
- the derived table contained a constant column.

The problem appeared because the flag 'maybe-null' of the wrapper
Item_direct_view_ref constructed for the constant field of the mergeable
derived table was not set to 'true' on the second execution of the
prepared statement.

The patch always sets this flag properly when calling the function
Item_direct_view_ref::set_null_ref-table(). The latter is invoked in
Item_direct_view_ref constructor if it is created for some reference of
a constant column belonging to a mergeable derived table.

Approved by Oleksandr Byelkin <sanja@mariadb.com>