[Libguestfs] [PATCH nbdkit] tls-fallback: Always call next open function

This fixes a crash when using --filter=tls-fallback. It happens to only occur with my block_size patch series and is easily reproducible (see commit message). I believe the problem is generic although I couldn't work out how to trigger in unpatched nbdkit. tls-fallback filter has a scary comment which seems to indicate that calling the next open is wrong (unsafe?) in the !tls case. So I don't know if this change is really right. Rich.