boost non_central_chi_squared_distribution() exception.

[huapeng01016]

The following code used to give correct result (1.53) but now (1.77) throws exception:

          boost::math::non_central_chi_squared_distribution<double> nchi2(0.001, 100);
          double res = quantile(nchi2, 0.00000000000003659);

// use to return 5.8758639130601491, now throws exception

I traced the issue to line 476 in boost/math/distributions/non_central_chi_squared.hpp

instead

               if(guess == 0)
                  guess = tools::min_value<value_type>(); 

add the checking of guess for infinity fixes the issue

               if(guess == 0 || isinf(guess))
                  guess = tools::min_value<value_type>();