Fix NaN bugs when sigma_n rounds to zero.

Fixes #179, via @shadeops.

src/pbrt/cpu/integrators.cpp

@@ -1055,6 +1055,7 @@ SampledSpectrum VolPathIntegrator::Li(RayDifferential ray, SampledWavelengths &l
                             ClampZero(sigma_maj - mp.sigma_a - mp.sigma_s);
                         Float pdf = T_maj[0] * sigma_n[0];
                         beta *= T_maj * sigma_n / pdf;
+                        if (pdf == 0) beta = SampledSpectrum(0.f);
                         r_u *= T_maj * sigma_n / pdf;
                         r_l *= T_maj * sigma_maj / pdf;
                         return beta && r_u;

src/pbrt/wavefront/media.cpp

@@ -128,6 +128,7 @@ void WavefrontPathIntegrator::SampleMediumInteraction(int wavefrontDepth) {
 
                         Float pr = T_maj[0] * sigma_n[0];
                         beta *= T_maj * sigma_n / pr;
+                        if (pr == 0) beta = SampledSpectrum(0.f);
                         inv_w_u *= T_maj * sigma_n / pr;
                         inv_w_l *= T_maj * sigma_maj / pr;