Improve FourierBSDFTable initialization.
Improve FourierBSDFTable initialization.

By setting nChannels to zero at the start and in failure cases, we avoid an
uninitialized memory read (and potentially, a crash), when the BSDF file
can't be read.

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