Ensure the pathspec is valid.

def Validate(self):
    """Ensure the pathspec is valid."""
    self.pathspec.Validate()

    if (self.HasField("start_time") and self.HasField("end_time") and
        self.start_time > self.end_time):
      raise ValueError("Start time must be before end time.")

    if not self.path_regex and not self.data_regex and not self.path_glob:
      raise ValueError("A Find specification can not contain both an empty "
                       "path regex and an empty data regex")