OBJ File format

[arvr007]

It crashes when the the minus facet index. see example
OBJ File

o BOX_@000053_0
mtllib cubeg.mtl
usemtl Mtl_1
v 0 1 1
v 0 1 0
v 0 0 0
v 0 0 1
v 1 1 1
v 1 1 0
v 0 1 1
v 1 0 1
v 1 0 0
vn 1 0 0
vn 0 -1 0
vn 0 0 -1
vn -1 0 0
f -9 -8 -7
f -9 -7 -6
f -5 -4 -8
f -5 -8 -3
f -2 -1 -4
f -2 -4 -5
f -6 -7 -1
f -6 -1 -2
f -3 -6 -2
f -3//-2 -2//-1 -5//-3
f -7 -8 -4
f -7 -4 -1

MTL file

newmtl Mtl_1
Kd 0.8 0.47450980392157 0.54509803921569
Tr 0
Ks 0.5 0.5 0.5
Ns 359.999993
Ni 1.45