MDEV-36860 exec_REDO_LOGREC_REDO_REPAIR_TABLE: Assertion `maria_tmpdir' failed

The problem was that aria_backup_client code did not intialize
maria_tmpdir, which is used during recovery if repair table is needed to
reconstruct indexes.

Author: montywi

extra/mariabackup/aria_backup_client.cc

@@ -955,6 +955,7 @@ void Backup::set_post_copy_table_hook(const post_copy_table_hook_t &hook) {
 
 bool prepare(const char *target_dir) {
 	maria_data_root= (char *)target_dir;
+        maria_tmpdir= &mysql_tmpdir_list;
 
 	if (maria_init())
 		die("Can't init Aria engine (%d)", errno);