MDEV-15988 Crash in ./mtr mariabackup.undo_space_id

xb_assign_undo_space_start(): Correctly pass the length of
the buffer, so that the file name will not be truncated.

extra/mariabackup/xtrabackup.cc

@@ -3107,7 +3107,7 @@ static dberr_t xb_assign_undo_space_start()
 		name[dirnamelen++] = SRV_PATH_SEPARATOR;
 	}
 
-	ut_snprintf(name + dirnamelen, strlen(name) + strlen("ibdata1"),
+	ut_snprintf(name + dirnamelen, (sizeof name) - dirnamelen,
 		    "%s", "ibdata1");
 
 	file = os_file_create(innodb_file_data_key, name, OS_FILE_OPEN,