some adjustment to deal with numerical errors

approximate at extreme nagetive:
+ norminv(u*normcdf(a)) = a if a << 0
+ normpdf(a)/normcdf(a) = -a if a << 0

some other guess
+ d_a_beta = 0 when d_a_beta == NaN
+ d_L_beta = 0 when d_L_beta == NaN
+ d_L_s = 0 when d_L_s == NaN
+ d_nLoglIke = 0 when d_nLogLIke == NaN