In Item_ref::Item_ref set maybe_null (and other fields fix_fields sets) to be the 
same as in (*ref), because Item_ref::fix_fields() will not be called.  Previously 
maybe_null was 0 always and this produced a bogus state where 
maybe_null==0 && is_null() == true 
which broke evaluation for some upper-level Items, like AND and OR.


mysql-test/r/group_by.result:
  Test for BUG#6976
mysql-test/t/group_by.test:
  Test for BUG#6976
sql/item.cc:
  Comment added
sql/item.h:
  Fix for BUG#6976:
    in Item_ref::Item_ref(Item**...) fix all fields because fix_fields() will not be called.