vfio-ccw: Don't assume there are more ccws after a TIC

When trying to calculate the length of a ccw chain, we assume
there are ccws after a TIC. This can lead to overcounting and
copying garbage data from guest memory.

Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
Message-Id: <d63748c1f1b03147bcbf401596638627a5e35ef7.1548082107.git.alifm@linux.ibm.com>
Reviewed-by: Halil Pasic <pasic@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>