Fix a rare NaN bug in HomogeneousMedium::Sample()
Fix a rare NaN bug in HomogeneousMedium::Sample()

If we get an infinite value back from the std::log call initializing dist, then we end up with a zero Tr value, and in turn a zero pdf. Catch this case and don't divide 0.f/0.f.

shihchinw/pbrt-v3,nispaur/pbrt-v3,ambakshi/pbrt-v3,nyue/pbrt-v3,shihchinw/pbrt-v3,mwkm/pbrt-v3,ambakshi/pbrt-v3,bssrdf/pbrt-v3,nyue/pbrt-v3,nyue/pbrt-v3,mmp/pbrt-v3,mmp/pbrt-v3,ambakshi/pbrt-v3,bssrdf/pbrt-v3,mmp/pbrt-v3,bssrdf/pbrt-v3,nyue/pbrt-v3,mmp/pbrt-v3,nispaur/pbrt-v3,shihchinw/pbrt-v3,nispaur/pbrt-v3,mwkm/pbrt-v3,nispaur/pbrt-v3,shihchinw/pbrt-v3