Fixed bug mdev-4369.

The function was adjusted to be able to aggregate
the counters of the merged elements. 
Before this change it was not possible to guarantee the correctness
of the counters passed to the the call-back parameter walk_action.
As a result, when some elements of a Unique object were flushed into
disk the function passed to merge_walk() as the call-back parameter
could return wrong counters of elements. This could lead to building
wrong histograms.

sql/sql_class.h

@@ -3997,6 +3997,7 @@ class Unique :public Sql_alloc
   uint size;
   uint full_size;
   uint min_dupl_count;   /* always 0 for unions, > 0 for intersections */
+  bool with_counters;
 
   bool merge(TABLE *table, uchar *buff, bool without_last_merge);