Fixed a rare case of NaN in Cycles

This fixes a rare case where NaNs could exist inside Cycles.

When certain invalid meshes were passed in, Cycles would try too normalize
a zero length normal during its setup stage. While it does check against
division by zero, it still returns a zero length normal and passes it on to
the path tracing kernel. The kernel then operates under the assumption that
normals are valid, and in the case of such a zero length normal, would
eventually create NaNs that propagate through and result in black pixels.

Reviewers: #cycles

Subscribers: brecht, sergey

Projects: #cycles

Differential Revision: https://developer.blender.org/D2008

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