erf(NaN) and erfc(NaN) do not return NaN

[jzmaddock]

It is expected that erf(NaN) and erfc(NaN) are NaN, but they are not:

#include <iostream>
#include <cmath>
#include "boost/math/special_functions/erf.hpp"

int main(){
    std::cout << boost::math::erf(nan("")) << "\n";
    std::cout << boost::math::erfc(nan("")) << "\n";
    return 0;
}

Output:

1
0

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