Fixed partial derivative NaN error for hair (#60)

src/shapes/hair.cpp

@@ -846,6 +846,7 @@ void HairShape::fillIntersectionRecord(const Ray &ray,
     its.p += its.geoFrame.n * (m_kdtree->getRadius() - std::sqrt(local.y*local.y+local.z*local.z));
 
     its.shFrame.n = its.geoFrame.n;
+    coordinateSystem(its.shFrame.n, its.dpdu, its.dpdv);
     its.hasUVPartials = false;
     its.instance = this;
     its.time = ray.time;