DAFS: Avoid volserver segfault in GetVolInfo

DAFS: Avoid volserver segfault in GetVolInfo

It is possible for GetVolObject to return success, but result in a NULL
fs_tv. In particular, when the fileserver responds with FSYNC_WRONG_PART
or FSYNC_UNKNOWN_VOLID. It is possible, though difficult, for the
fileserver to respond with FSYNC_UNKNOWN_VOLID if the requested volume
hasn't been preattached on the fileserver yet.

So, in GetVolInfo, also check fs_tv, so we don't try to dereference it
later.

Reviewed-on: http://gerrit.openafs.org/562
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>