MDEV-11436 CREATE TABLE .. AS SELECT JSON_OBJECT truncates data.

fix_length_and_dec() didn't calculate the length of the result
        properly.

mysql-test/r/func_json.result

@@ -165,6 +165,16 @@ json_object()
 select json_object("ki", 1, "mi", "ya");
 json_object("ki", 1, "mi", "ya")
 {"ki": 1, "mi": "ya"}
+create table t1 as select json_object('id', 87, 'name', 'carrot') as f;
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `f` varchar(32) NOT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+select * from t1;
+f
+{"id": 87, "name": "carrot"}
+drop table t1;
 select json_exists('{"key1":"xxxx", "key2":[1, 2, 3]}', "$.key2");
 json_exists('{"key1":"xxxx", "key2":[1, 2, 3]}', "$.key2")
 1

mysql-test/t/func_json.test

@@ -68,6 +68,10 @@ select json_remove(@j, '$.a');
 
 select json_object();
 select json_object("ki", 1, "mi", "ya");
+create table t1 as select json_object('id', 87, 'name', 'carrot') as f;
+show create table t1;
+select * from t1;
+drop table t1;
 
 select json_exists('{"key1":"xxxx", "key2":[1, 2, 3]}', "$.key2");
 select json_exists('{"key1":"xxxx", "key2":[1, 2, 3]}', "$.key2[1]");

sql/item_jsonfunc.cc

@@ -874,7 +874,7 @@ append_null:
 
 void Item_func_json_array::fix_length_and_dec()
 {
-  ulonglong char_length= 4;
+  ulonglong char_length= 2;
   uint n_arg;
 
   if (arg_count == 0)
@@ -889,7 +889,7 @@ void Item_func_json_array::fix_length_and_dec()
     return;
 
   for (n_arg=0 ; n_arg < arg_count ; n_arg++)
-    char_length+= args[n_arg]->max_char_length() + 2;
+    char_length+= args[n_arg]->max_char_length() + 4;
 
   fix_char_length_ulonglong(char_length);
   tmp_val.set_charset(collation.collation);