The symptoms of this bug is sometimes an assertion, sometimes
only a valgrind warning about "jump or move depends on uninitialised
value" in do_copy_null(). The problem may occur when a derived table
with an ORDER BY clause is merged into an outer query, when the columns
from ORDER BY are not also referenced in outer query. The problem is
due to missing to mark the column underlying "field2" in the read set
of table t1.

field2 is used in ORDER BY inside a derived table. field2 references
a column from the SELECT list. But since fields from SELECT list
in a derived table are not marked in the read_set when they are first
resolved, we need to make sure those fields are marked when used for
other purposes, such as in ORDER BY. Notice that we only want to
mark columns when merging into an outermost query expression, and
not into another derived table.

Bug case is added to main.derived-no-prepared due to bug#26808862.