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

[Richard W.M. Jones]

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.