MDEV-26626 fixup: SIGFPE during startup

srv_start(): Set srv_startup_is_before_trx_rollback_phase before
starting the buf_flush_page_cleaner() thread, so that it will not
invoke log_checkpoint() before the log file has been created.

This race condition was reproduced with https://rr-project.org.
This fixes up commit 15efb7ed

storage/innobase/srv/srv0start.cc

@@ -1243,14 +1243,13 @@ dberr_t srv_start(bool create_new_db)
 	recv_sys.create();
 	lock_sys.create(srv_lock_table_size);
 
+	srv_startup_is_before_trx_rollback_phase = true;
 
 	if (!srv_read_only_mode) {
 		buf_flush_page_cleaner_init();
 		ut_ad(buf_page_cleaner_is_active);
 	}
 
-	srv_startup_is_before_trx_rollback_phase = true;
-
 	/* Check if undo tablespaces and redo log files exist before creating
 	a new system tablespace */
 	if (create_new_db) {