The patch caused some test failures when merged to 5.5 because,
unlike 5.1, it utilizes Item_cache_row to actually cache row
values. The problem was that Item_cache_row::bring_value()
essentially did nothing. In particular, it did not update its
null_value, so all Item_cache_row objects were always having
their null_values set to TRUE. This went unnoticed previously,
but now when Arg_comparator::compare_row() actually depends on
the row's null_value to evaluate the comparison, the problem
has surfaced.

Fixed by calling the underlying item's bring_value() and
updating null_value in Item_cache_row::bring_value().

Since the problem also exists in 5.1 code (albeit hidden, since
the relevant code is not used anywhere), the addendum patch is
against 5.1.