Fix assertion when there is no normal/texcoord assigned to the face. Fixes #161.

Suppress clang warnings.

models/issue-161-inconsistent-f.obj

@@ -0,0 +1,37 @@
+o cube
+mtllib cube.mtl
+
+v -0.500000 -0.500000 0.500000
+v 0.500000 -0.500000 0.500000
+v -0.500000 0.500000 0.500000
+v 0.500000 0.500000 0.500000
+v -0.500000 0.500000 -0.500000
+v 0.500000 0.500000 -0.500000
+v -0.500000 -0.500000 -0.500000
+v 0.500000 -0.500000 -0.500000
+
+vt 0.000000 0.000000
+vt 1.000000 0.000000
+vt 0.000000 1.000000
+vt 1.000000 1.000000
+
+g cube
+usemtl cube
+s 1
+f 1/1 2/2 3/3
+f 3/3 2/2 4/4
+s 2
+f 3/1 4/2 5/3
+f 5/3 4/2 6/4
+s 3
+f 5/4 6/3 7/2
+f 7/2 6/3 8/1
+s 4
+f 7/1 8/2 1/3
+f 1/3 8/2 2/4
+s 5
+f 2/1 8/2 4/3
+f 4/3 8/2 6/4
+s 6
+f 7/1 1/2 5/3
+f 5 1 3