Return a GrrMessage instance from the transaction log or None.

def Get(self):
    """Return a GrrMessage instance from the transaction log or None."""
    try:
      with io.open(self.logfile, "rb") as fd:
        data = fd.read(self.max_log_size)
    except (IOError, OSError):
      return

    try:
      if data:
        return rdf_flows.GrrMessage.FromSerializedString(data)
    except (message.Error, rdfvalue.Error):
      return