SeedLink: Strip trailing NULL bytes from INFO responses

SeedLink INFO responses consist of 512 Byte long Mini-SEED data, zero padded. This padding is not stripped from responses.

To see the output:

```sh
$ python slclient.py -i ID geofon.gfz-potsdam.de:18000 | hexdump
```

This is fixed in 459c5cc (0158148 for the master branch).