Cycles: Fix crash after recent subd fixes

Was happening when object only had curves (doe example, object with hair
particle system and emitter rendering disabled).

intern/cycles/render/mesh.cpp

@@ -587,6 +587,10 @@ void Mesh::add_vertex_normals()
 void Mesh::pack_normals(Scene *scene, uint *tri_shader, float4 *vnormal)
 {
 	Attribute *attr_vN = attributes.find(ATTR_STD_VERTEX_NORMAL);
+	if(attr_vN == NULL) {
+		/* Happens on objects with just hair. */
+		return;
+	}
 
 	float3 *vN = attr_vN->data_float3();
 	uint shader_id = 0;