In commit 83d2e084 (MDEV-24041)
we failed to notice that in addition to the bug with
DELETE and ON DELETE CASCADE, there is another bug with
UPDATE and ON UPDATE CASCADE.

row_ins_foreign_fill_virtual(): Use the correct memory heap
for everything that will be reachable from the cascade->update
that we return to the caller.

Note: It is correct to use the shorter-lived cascade->heap for
rec_get_offsets(), because that memory will be abandoned when
row_ins_foreign_fill_virtual() returns.