doubled slash in paths in error log

```
2019.12.19 17:33:18.401719 [ 1 ] {} <Error> Application: DB::Exception: Cannot parse definition from metadata file /var/lib/clickhouse/metadata/default//aaaa.sql, error: DB::Exception: Syntax error (in file /var/lib/clickhouse/metadata/default//aaaa.sql): failed at position 21 (line 3, col 1): )
```

etc