log_frame() only can handle frames with 8 data bytes

[DfePeterWinkler]

in log.py::log_frame()[l.35] is assumed that the frame contains 8 data bytes. IMHO CAN messages might be lower than 8 data bytes esp. 0 data bytes. So I would expect lines something like:
line = ("(%f) %s %03X#" % (ts, self.if_name, frame.arb_id))
line += "".join(["%02X" % x for x in frame.data])
line += "\n"