warning for valid compressed partial WAL segment

Rather silly issue that is easy to reproduce:

```sh
# note the -Z option
pg_receivewal -Z 1 -D $backup/wal/$instance -h … &
pg_probackup delete -B $backup --instance $instance --delete-wal
```

Warning in question:

```
WARNING: unexpected WAL file name "00000001000003C30000006C.gz.partial"
```

I'm running PostgreSQL 11, if that matters.