Fix bug in HomogeneousMedium::Sample()
Fix bug in HomogeneousMedium::Sample()

Reported by Srinath Ravichandiran, also issue #159.

nyue/pbrt-v3,nyue/pbrt-v3,nyue/pbrt-v3,mmp/pbrt-v3,mmp/pbrt-v3,mmp/pbrt-v3,mmp/pbrt-v3,nyue/pbrt-v3