backup: ensure base snapshots are still available after backup

This should never trigger if everything else works correctly, but it is
still a very cheap check to avoid wrongly marking a backup as "OK" when
in fact some chunks might be missing.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>

src/api2/backup.rs

@@ -660,6 +660,7 @@ fn download_previous(
             };
             if let Some(index) = index {
                 env.log(format!("register chunks in '{}' from previous backup.", archive_name));
+                env.register_base_snapshot(last_backup.backup_dir.clone());
 
                 for pos in 0..index.index_count() {
                     let info = index.chunk_info(pos).unwrap();