Fixed memory leak when using histograms

This was introduced in last merge with 10.6
The reason is that 10.6 does not need anything special to free histograms
as everything is allocated on a memroot.
In 10.10 histograms is using the vector class, which has some problems:
- No automatic free
- No memory usage accounting
(we should at some point remove vector usage because of the above problem)

Fixed by expliciting freeing histograms when freeing TABLE_STATISTICS
objects.

sql/sql_statistics.h

@@ -432,9 +432,9 @@ class Index_statistics;
 
 class Table_statistics
 {
-
 public:
   my_bool cardinality_is_null;      /* TRUE if the cardinality is unknown */
+  uint  columns;                    /* Number of columns in table */
   ha_rows cardinality;              /* Number of rows in the table        */
   uchar *min_max_record_buffers;    /* Record buffers for min/max values  */
   Column_statistics *column_stats;  /* Array of statistical data for columns */