Fix to prevent undefined normals being generated with `add_face_normals()', from triangles with zero area.

intern/cycles/render/mesh.cpp

@@ -178,7 +178,12 @@ void Mesh::add_face_normals()
 			float3 v1 = verts_ptr[t.v[1]];
 			float3 v2 = verts_ptr[t.v[2]];
 
-			fN[i] = normalize(cross(v1 - v0, v2 - v0));
+			float3 norm = cross(v1 - v0, v2 - v0);
+			float normlen = len(norm);
+			if(normlen == 0.0f)
+				fN[i] = make_float3(0.0f, 0.0f, 0.0f);
+			else
+				fN[i] = norm / normlen;
 
 			if(flip)
 				fN[i] = -fN[i];