Fix garbage null values at end of histogram json

Signed-off-by: Michael Okoko <okokomichaels@outlook.com>

mysql-test/main/statistics_json.result

@@ -30,7 +30,7 @@ test	t1	a	1	25	0.0000	4.0000	1.0000	10	JSON	[
   "19",
   "21",
   "23"
-]
+]
 test	t1	b	1	9	0.0000	1.6400	1.0000	10	JSON	[
   "11",
   "13",
@@ -42,7 +42,7 @@ test	t1	b	1	9	0.0000	1.6400	1.0000	10	JSON	[
   "3",
   "5",
   "7"
-]
+]
 test	t1	c	1	9	0.0000	2.0000	1.0000	10	JSON	[
   "11",
   "13",
@@ -54,7 +54,7 @@ test	t1	c	1	9	0.0000	2.0000	1.0000	10	JSON	[
   "3",
   "5",
   "7"
-]
+]
 test	t1	d	1	25	0.0000	8.0000	1.0000	10	JSON	[
   "3",
   "5",
@@ -66,7 +66,7 @@ test	t1	d	1	25	0.0000	8.0000	1.0000	10	JSON	[
   "19",
   "21",
   "23"
-]e
+]
 DELETE FROM mysql.column_stats;
 DROP TABLE t1;
 create schema world;
@@ -102,7 +102,7 @@ Code	ABW	ZWE	25	JSON	[
   "TMP",
   "UKR",
   "VIR"
-]0
+]
 Name	Afghanistan	Zimbabwe	25	JSON	[
   "Argentina",
   "Barbados",
@@ -129,7 +129,7 @@ Name	Afghanistan	Zimbabwe	25	JSON	[
   "Togo",
   "Uganda",
   "Venezuela"
-]a
+]
 SurfaceArea	0.40	17075400.00	25	JSON	[
   "36.00",
   "151.00",
@@ -156,7 +156,7 @@ SurfaceArea	0.40	17075400.00	25	JSON	[
   "1104300.00",
   "1648195.00",
   "2724900.00"
-]
+]
 Population	0	1277558000	25	JSON	[
   "1000",
   "8000",
@@ -183,7 +183,7 @@ Population	0	1277558000	25	JSON	[
   "40377000",
   "61399000",
   "111506000"
-]
+]
 Capital	1	4074	25	JSON	[
   "62",
   "149",
@@ -210,7 +210,7 @@ Capital	1	4074	25	JSON	[
   "3336",
   "3426",
   "3538"
-]

+]
 ID	1	4079	10	JSON	[
   "371",
   "742",
@@ -222,7 +222,7 @@ ID	1	4079	10	JSON	[
   "2967",
   "3338",
   "3709"
-]
+]
 Name	A Coruña (La Coruña)	Århus	10	JSON	[
   "Berdytšiv",
   "Clermont-Ferrand",
@@ -234,7 +234,7 @@ Name	A Coruña (La Coruña)	Århus	10	JSON	[
   "Roanoke",
   "Split",
   "Ulsan"
-]
+]
 Country	ABW	ZWE	10	JSON	[
   "BRA",
   "CHN",
@@ -246,7 +246,7 @@ Country	ABW	ZWE	10	JSON	[
   "PHL",
   "RUS",
   "USA"
-]
+]
 Population	42	10500000	10	JSON	[
   "96002",
   "105819",
@@ -258,7 +258,7 @@ Population	42	10500000	10	JSON	[
   "288173",
   "398300",
   "670208"
-]
+]
 Country	ABW	ZWE	10	JSON	[
   "BGD",
   "CMR",
@@ -270,7 +270,7 @@ Country	ABW	ZWE	10	JSON	[
   "PNG",
   "SVK",
   "UGA"
-]
+]
 Language	Abhyasi	[South]Mande	10	JSON	[
   "Bariba",
   "Creole English",
@@ -294,7 +294,7 @@ Percentage	0.0	99.9	10	JSON	[
   "17.7",
   "45.5",
   "87.5"
-]q
+]
 set histogram_type=@save_histogram_type;
 set histogram_size=@save_histogram_size;
 DROP SCHEMA world;

sql/sql_statistics.cc

@@ -1643,7 +1643,8 @@ std::vector<std::string> bucket_bounds;
     }
     writer->end_array();
     histogram->set_size(bucket_bounds.size());
-    histogram->set_values((uchar *) writer->output.get_string()->ptr());
+    Binary_string *json_string = (Binary_string *) writer->output.get_string();
+    histogram->set_values((uchar *) json_string->c_ptr());
   }
 };