MDEV-11437 JSON_QUOTE function does not quote and uses wrong character set.

        json_quote fixed.

mysql-test/r/func_json.result

@@ -186,7 +186,17 @@ json_exists('{"key1":"xxxx", "key2":[1, 2, 3]}', "$.key2[10]")
 0
 select json_quote('"string"');
 json_quote('"string"')
-\"string\"
+"\"string\""
+create table t1 as select json_quote('foo');
+select * from t1;
+json_quote('foo')
+"foo"
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `json_quote('foo')` varchar(10) CHARACTER SET utf8mb4 COLLATE utf8mb4_bin NOT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+drop table t1;
 select json_merge('string', 123);
 json_merge('string', 123)
 ["string", 123]

mysql-test/t/func_json.test

@@ -78,6 +78,10 @@ select json_exists('{"key1":"xxxx", "key2":[1, 2, 3]}', "$.key2[1]");
 select json_exists('{"key1":"xxxx", "key2":[1, 2, 3]}', "$.key2[10]");
 
 select json_quote('"string"');
+create table t1 as select json_quote('foo');
+select * from t1;
+show create table t1;
+drop table t1;
 
 select json_merge('string', 123);
 

sql/item_jsonfunc.cc

@@ -287,12 +287,12 @@ bool Item_func_json_query::check_and_get_value(json_engine_t *je, String *res,
 
 void Item_func_json_quote::fix_length_and_dec()
 {
-  collation.set(args[0]->collation);
+  collation.set(&my_charset_utf8mb4_bin);
   /*
     Odd but realistic worst case is when all characters
     of the argument turn into '\uXXXX\uXXXX', which is 12.
   */
-  max_length= args[0]->max_length * 12;
+  max_length= args[0]->max_length * 12 + 2;
 }
 
 
@@ -300,13 +300,16 @@ String *Item_func_json_quote::val_str(String *str)
 {
   String *s= args[0]->val_str(&tmp_s);
 
-  if ((null_value= args[0]->null_value))
+  if ((null_value= (args[0]->null_value ||
+                    args[0]->result_type() != STRING_RESULT)))
     return NULL;
 
   str->length(0);
-  str->set_charset(s->charset());
+  str->set_charset(&my_charset_utf8mb4_bin);
 
-  if (st_append_escaped(str, s))
+  if (str->append("\"", 1) ||
+      st_append_escaped(str, s) ||
+      str->append("\"", 1))
   {
     /* Report an error. */
     null_value= 1;