Error with `non_central_chi_squared::find_degrees_of_freedom` when running with Valgrind

Hello,

I ported the function `non_central_chi_squared::find_degrees_of_freedom` to a R package. My R-C++ function is just:

```cpp
double find_chisq_df(double delta, double q, double p){
// gives nu such that pchisq(q, nu, delta) = p
return non_central_chi_squared::find_degrees_of_freedom(delta, q, p);
}
```

When an example is executed under Valgrind, this error is generated:

```
Error in function boost::math::non_central_chi_squared_distribution::non_central_chi_squared_distribution(long double,long double): Degrees of freedom argument is -nan, but must be > 0 !
```

Would you have an idea of the cause? I'm really not fluent in C++.