Incorrect pointer addition in SpectralFilm can lead to out-of-bounds memory access.

[mpriestman]

In the constructor for SpectralFilm, the splatBuffer pointer is incremented by NSpectrumSamples. I think this is wrong, and it should be incremented by nBuckets instead.

https://github.com/mmp/pbrt-v4/blob/master/src/pbrt/film.cpp#L885

When nBuckets is greater than NSpectrumSamples, then no crash, but I think you'll get incorrect behaviour as the bucketSplats arrays will be overlapping. If nBuckets is less than NSpectrumSamples, then you'll end up reading/writing off the end of the allocated memory.