Valgrind fixes, poor .result fixes, code cleanups

- Use String::c_ptr_safe() instead of String::c_ptr
- Do proper datatype conversions in Histogram_json_hb::parse
- Remove Histogram_json_hb::Bucket::end_value. Introduce
  get_end_value() instead.

mysql-test/main/statistics_json.result

@@ -4013,12 +4013,12 @@ test	t1_json	a	a-0	a-9	0.0000	3.0000	1.0000	10	JSON_HB	{
 }
 explain extended select * from t1_json where a between 'a-3a' and 'zzzzzzzzz';
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
-1	SIMPLE	t1_json	ALL	NULL	NULL	NULL	NULL	10	10.00	Using where
+1	SIMPLE	t1_json	ALL	NULL	NULL	NULL	NULL	10	50.00	Using where
 Warnings:
 Note	1003	select `test`.`t1_json`.`a` AS `a` from `test`.`t1_json` where `test`.`t1_json`.`a` between 'a-3a' and 'zzzzzzzzz'
 analyze select * from t1_json where a between 'a-3a' and 'zzzzzzzzz';
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	r_rows	filtered	r_filtered	Extra
-1	SIMPLE	t1_json	ALL	NULL	NULL	NULL	NULL	10	10.00	10.00	60.00	Using where
+1	SIMPLE	t1_json	ALL	NULL	NULL	NULL	NULL	10	10.00	50.00	60.00	Using where
 explain extended select * from t1_json where a < 'b-1a';
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t1_json	ALL	NULL	NULL	NULL	NULL	10	90.00	Using where
@@ -4053,12 +4053,12 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	r_rows	filtered	r_f
 1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	101	101.00	50.00	98.02	Using where
 explain extended select * from t2 where city = 'Helsinki';
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
-1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	101	50.00	Using where
+1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	101	1.98	Using where
 Warnings:
 Note	1003	select `test`.`t2`.`city` AS `city` from `test`.`t2` where `test`.`t2`.`city` = 'Helsinki'
 analyze select * from t2 where city = 'helsinki';
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	r_rows	filtered	r_filtered	Extra
-1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	101	101.00	50.00	1.98	Using where
+1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	101	101.00	1.98	1.98	Using where
 explain extended select * from t2 where city < 'Lagos';
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	101	50.00	Using where

sql/opt_histogram_json.h

@@ -61,9 +61,6 @@ class Histogram_json_hb : public Histogram_base
     // value is in this bucket.
     std::string start_value;
 
-    // The right endpoint. It is non-inclusive, except for the last bucket.
-    std::string *end_value;
-
     // Cumulative fraction: The fraction of table rows that fall into this
     //  and preceding buckets.
     double cum_fract;
@@ -125,6 +122,7 @@ class Histogram_json_hb : public Histogram_base
 
 private:
   double get_left_fract(int idx);
+  std::string& get_end_value(int idx);
   int find_bucket(Field *field, const uchar *lookup_val, bool equal_is_less);
 };