Omit some operations if only one para is kLogZero

If log_prob_1 is kLogZero and log_prob_2 is not kLogZero or log_prob_2 is kLogZero and log_prob_1 is not kLogZero, just return another parameters, this judge can omit some operations(log1pf() and expf()).
The mathematical equation is:
ln(e^(-inf)+e^(x))=ln(0+e^(x))=x