Bug#51980 mysqld service crashes with a simple COUNT(DISTINCT) query over a view

Problem: Segmentation fault in add_group_and_distinct_keys() when accessing
field of what is assumed to be an Item_field object.

Cause: In case of views, the item added to list by is_indexed_agg_distinct() 
was not of type Item_field, but Item_ref.

Resolution:  Add the real Item_field object, the one referred to by 
Item_ref object, to the list, instead.

mysql-test/r/count_distinct.result:
  Results for test case for Bug#51980.
mysql-test/t/count_distinct.test:
  Test case for Bug#51980.
  Table needs to contain at least two rows to avoid const table optimization.
sql/sql_select.cc:
  Make sure it is the actual Item_field object that is pushed to the out_args
  list of is_indexed_agg_distinct(), and not Item_ref objects.

mysql-test/r/count_distinct.result

@@ -86,3 +86,11 @@ select count(distinct if(f1,3,f2)) from t1;
 count(distinct if(f1,3,f2))
 2
 drop table t1;
+create table t1 (i int);
+insert into t1 values (0), (1);
+create view v1 as select * from t1;
+select count(distinct i) from v1;
+count(distinct i)
+2
+drop table t1;
+drop view v1;

mysql-test/t/count_distinct.test

@@ -96,3 +96,14 @@ insert into t1 values (0,1),(1,2);
 select count(distinct if(f1,3,f2)) from t1;
 drop table t1;
 
+#
+# Bug #51980 "mysqld service crashes with a simple COUNT(DISTINCT) query 
+#             over a view"
+#
+
+create table t1 (i int);
+insert into t1 values (0), (1);
+create view v1 as select * from t1;
+select count(distinct i) from v1;
+drop table t1;
+drop view v1;

sql/sql_select.cc

@@ -4157,7 +4157,7 @@ is_indexed_agg_distinct(JOIN *join, List<Item_field> *out_args)
         optimization is applicable 
       */
       if (out_args)
-        out_args->push_back((Item_field *) expr);
+        out_args->push_back((Item_field *) expr->real_item());
       result= true;
     }
   }