More "straightforward" memory management

Do not put Histogram objects on MEM_ROOT at all

sql/sql_statistics.cc

@@ -1238,6 +1238,8 @@ class Column_stat: public Stat_table
         table_field->read_stats->histogram= hist;
         return hist;
       }
+      else
+        delete hist;
     }
     return NULL;
   }
@@ -1647,10 +1649,10 @@ Histogram_base *create_histogram(MEM_ROOT *mem_root, Histogram_type hist_type,
   switch (hist_type) {
   case SINGLE_PREC_HB:
   case DOUBLE_PREC_HB:
-    res= new (mem_root) Histogram_binary();
+    res= new Histogram_binary();
     break;
   case JSON_HB:
-    res= new (mem_root) Histogram_json_hb();
+    res= new Histogram_json_hb();
     break;
   default:
     DBUG_ASSERT(0);

sql/sql_statistics.h

@@ -151,7 +151,7 @@ class Histogram_builder;
 /*
   Common base for all histograms
 */
-class Histogram_base : public Sql_alloc
+class Histogram_base
 {
 public:
   virtual bool parse(MEM_ROOT *mem_root, Field *field, Histogram_type type_arg,