MDEV-17479 Assertion `mysql_socket.fd != -1' failed in inline_mysql_socket_send on server shutdown

Do not try to write ER_SHUTDOWN error message to socket, when it is forcefully closed by the shutdown.
This will avoid the race condition (attempt to write to closed socket,  if connection shuts down by itself).

sql/mysqld.cc

@@ -1791,7 +1791,7 @@ static void close_connections(void)
       */
       THD* save_thd= current_thd;
       set_current_thd(tmp);
-      close_connection(tmp,ER_SERVER_SHUTDOWN);
+      close_connection(tmp);
       set_current_thd(save_thd);
     }
 #endif