MDEV-13435 Crash when selecting virtual columns generated using JSON functions

don't allocate memory on thd->mem_root in every fix_fields(),
do it once and reuse.

mysql-test/suite/vcol/r/wrong_arena.result

@@ -59,3 +59,12 @@ a	b
 Warnings:
 Warning	1292	Incorrect datetime value: '1'
 drop table t1;
+create table t1 (
+id int  not null ,
+js varchar(1000) not null,
+t time AS (cast(json_value(json_extract(js,concat('$.singleDay."', dayname(curdate()),'"')),'$.start') as time)) virtual);
+insert  into t1(id,js) values (0, '{"default" : {"start": "00:00:00", "end":"23:59:50"}}');
+select * from t1;
+id	js	t
+0	{"default" : {"start": "00:00:00", "end":"23:59:50"}}	NULL
+drop table t1;

mysql-test/suite/vcol/t/wrong_arena.test

@@ -37,3 +37,14 @@ disconnect con1;
 connection default;
 select * from t1;
 drop table t1;
+
+#
+# MDEV-13435 Crash when selecting virtual columns generated using JSON functions
+#
+create table t1 (
+  id int  not null ,
+  js varchar(1000) not null,
+  t time AS (cast(json_value(json_extract(js,concat('$.singleDay."', dayname(curdate()),'"')),'$.start') as time)) virtual);
+insert  into t1(id,js) values (0, '{"default" : {"start": "00:00:00", "end":"23:59:50"}}');
+select * from t1;
+drop table t1;

sql/item_jsonfunc.cc

@@ -643,17 +643,21 @@ String *Item_func_json_unquote::val_str(String *str)
 
 
 static int alloc_tmp_paths(THD *thd, uint n_paths,
-                           json_path_with_flags **paths,String **tmp_paths)
+                           json_path_with_flags **paths, String **tmp_paths)
 {
   if (n_paths > 0)
   {
-    *paths= (json_path_with_flags *) alloc_root(thd->mem_root,
+    if (*tmp_paths == 0)
+    {
+      MEM_ROOT *root= thd->stmt_arena->mem_root;
+      *paths= (json_path_with_flags *) alloc_root(root,
           sizeof(json_path_with_flags) * n_paths);
-    *tmp_paths= (String *) alloc_root(thd->mem_root, sizeof(String) * n_paths);
+      *tmp_paths= (String *) alloc_root(root, sizeof(String) * n_paths);
       if (*paths == 0 || *tmp_paths == 0)
         return 1;
 
       bzero(*tmp_paths, sizeof(String) * n_paths);
+    }
 
     return 0;
   }
@@ -687,7 +691,6 @@ void Item_json_str_multipath::cleanup()
   {
     for (uint i= get_n_paths(); i>0; i--)
       tmp_paths[i-1].free();
-    tmp_paths= 0;
   }
   Item_str_func::cleanup();
 }