ftmod_libpri: Fix possible NULL-ptr dereference crash in on_restart(_ack)()

Catch single channel RESTART/RESTART ACK events for non-B-channels,
that would lead to a NULL pointer dereference, because those do not have
per-channel private data (chan->call_data = chan_priv == NULL).

Signed-off-by: Stefan Knoblich <stkn@openisdn.net>