vtkOBJReader: avoid a possible null buffer access
The OBJ in issue #18762 (closed) has no vertex normals, yet its faces contain invalid reference numbers to them. This leads to a non-zero n_normal_pts and a null normals buffer in vtkOBJReader.
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The OBJ in issue #18762 (closed) has no vertex normals, yet its faces contain invalid reference numbers to them. This leads to a non-zero n_normal_pts and a null normals buffer in vtkOBJReader.