MDEV-26657 : Initialize some fields in create_background_thd()

Avoid reading uninitialized memory by  thd_get_error_context_description().
Note, that THD::real_id can't be initialized at this stage, so it will be zeroed.

sql/sql_class.cc

@@ -4922,6 +4922,9 @@ MYSQL_THD create_background_thd()
   thd->set_command(COM_DAEMON);
   thd->system_thread= SYSTEM_THREAD_GENERIC;
   thd->security_ctx->host_or_ip= "";
+  thd->real_id= 0;
+  thd->thread_id= 0;
+  thd->query_id= 0;
   return thd;
 }