Fix orennayarbrdf.cpp: no longer try to normalizes zero vectors.

[andreasdeclercq]

Calculate the norm of each vector first and do an early return if zero. This avoids asserts in debug mode and divisions by null in release mode.

[oktomus]

Thanks for your contribution @andreasdeclercq !

This avoids asserts in debug mode and divisions by null in release mode.

Can you please explain in which case you get divisions by null ? That would be great if you could provide a scene so that we can test.