MDEV-31995 Fix2 allocate memory in mem_root properly.

Lex_ident_sys had no new operator and was used incorrectly in
save_item_list_names(), so leaked memory.

sql/sql_lex.cc

@@ -10697,8 +10697,9 @@ bool st_select_lex::save_item_list_names(THD *thd)
 
   while ((item= li++))
   {
-    if (unlikely(orig_names_of_item_list_elems->push_back(
-          new Lex_ident_sys(item->name.str, item->name.length), thd->mem_root)))
+    Lex_ident_sys *name= new (thd->mem_root) Lex_ident_sys(thd, &item->name);
+    if (unlikely(!name ||
+          orig_names_of_item_list_elems->push_back(name,  thd->mem_root)))
     {
       if (arena)
         thd->restore_active_arena(arena, &backup);

sql/sql_lex.h

@@ -143,6 +143,11 @@ class Lex_ident_cli: public Lex_ident_cli_st
 struct Lex_ident_sys_st: public LEX_CSTRING
 {
 public:
+  static void *operator new(size_t size, MEM_ROOT *mem_root) throw ()
+  { return alloc_root(mem_root, size); }
+  static void operator delete(void *ptr,size_t size) { TRASH_FREE(ptr, size); }
+  static void operator delete(void *ptr, MEM_ROOT *mem_root) {}
+
   bool copy_ident_cli(THD *thd, const Lex_ident_cli_st *str);
   bool copy_keyword(THD *thd, const Lex_ident_cli_st *str);
   bool copy_sys(THD *thd, const LEX_CSTRING *str);
@@ -176,6 +181,10 @@ class Lex_ident_sys: public Lex_ident_sys_st
     LEX_CSTRING tmp= {name, length};
     set_valid_utf8(&tmp);
   }
+  Lex_ident_sys(THD *thd, const LEX_CSTRING *str)
+  {
+    set_valid_utf8(str);
+  }
   Lex_ident_sys & operator=(const Lex_ident_sys_st &name)
   {
     Lex_ident_sys_st::operator=(name);