resolve_const_item() assumed to be not called for Item_row items. For
ensuring that DBUG_ASSERT(0) was set there.

This patch adds section for Item_row items. If it can it recursively calls
resolve_const_item() for each item the Item_row contains. If any of the
contained items is null then whole Item_row substitued by Item_null. Otherwise
it just returns.


sql/item.cc:
  Fix bug#13356 resolve_const_item() wasn't able to handle Item_row items.
  
  Added section to resolve_const_item() for Item_row items. If it can it
  recursively calls resolve_const_item() for each item the Item_row contains. If
  any of the contained items is null then Item_row is substituted by Item_null.
  Otherwise it just returns.
  Comment moved closer to function it belongs to.
mysql-test/t/select.test:
  Test case for bug#13356 resolve_const_item() wasn't able to handle Item_row items.
mysql-test/r/select.result:
  Test case for bug#13356 resolve_const_item() wasn't able to handle Item_row items.