Bug#13734987 MEMORY LEAK WITH I_S/SHOW AND VIEWS WITH SUBQUERY

In fill_schema_table_by_open(): free item list before restoring active arena.

sql/sql_show.cc

@@ -3164,8 +3164,9 @@ end:
   /* Restore original LEX value, statement's arena and THD arena values. */
   lex_end(thd->lex);
 
-  if (i_s_arena.free_list)
-    i_s_arena.free_items();
+  // Free items, before restoring backup_arena below.
+  DBUG_ASSERT(i_s_arena.free_list == NULL);
+  thd->free_items();
 
   /*
     For safety reset list of open temporary tables before closing