Fix subtle bug in FourierBSDF::Sample_f().
Fix subtle bug in FourierBSDF::Sample_f().

Computed wi directions could end up being far from normalized in certain
circumstances.

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