MDEV-13970 crash in Item_func_json_extract::read_json.

        Item_func_json_extract::val_int fixed.
        It wasn't tested yet as it's called in exotic cases only.

mysql-test/r/func_json.result

@@ -731,3 +731,8 @@ Warning	4042	Syntax error in JSON path in argument 3 to function 'json_contains_
 select JSON_VALID(0x36f0c8dccd83c5eac156da);
 JSON_VALID(0x36f0c8dccd83c5eac156da)
 0
+create table t1(a double not null);
+insert into t1 values (2),(1);
+select 1 from t1 where json_extract(a,'$','$[81]');
+1
+drop table t1;

mysql-test/t/func_json.test

@@ -383,3 +383,12 @@ select json_contains_path('{"foo":"bar"}', 'one', '$[]');
 # MDEV-13971 crash in skip_num_constant.
 #
 select JSON_VALID(0x36f0c8dccd83c5eac156da);
+
+#
+# MDEV-13970 crash in Item_func_json_extract::read_json.
+#
+create table t1(a double not null);
+insert into t1 values (2),(1);
+select 1 from t1 where json_extract(a,'$','$[81]');
+drop table t1;
+

sql/item_jsonfunc.cc

@@ -781,10 +781,10 @@ String *Item_func_json_extract::read_json(String *str,
   {
     str->set_charset(js->charset());
     str->length(0);
-  }
 
     if (possible_multiple_values && str->append("[", 1))
       goto error;
+  }
 
   json_get_path_start(&je, js->charset(),(const uchar *) js->ptr(),
                       (const uchar *) js->ptr() + js->length(), &p);