Fix loading of OBJ file with minus index.
Fix loading of OBJ file with minus index.

google/draco,google/draco,google/draco,google/draco,louquillio/draco,google/draco,google/draco,google/draco,louquillio/draco,louquillio/draco,louquillio/draco,louquillio/draco,louquillio/draco