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.

Warnings:

• please consider adding a changelog entry in a file ending with .md in Documentation/release/dev.

The warnings do not need to be fixed, but it is recommended to do so.

@mwestphal could you please help review it?

+2

Do: test

Dashboard is clean, merging

Do: merge

mentioned in commit b9c73f3c

mentioned in issue #18762 (closed)

merged

mentioned in commit thomas.galland/paraview@5fca5be9

mentioned in merge request paraview/paraview!6113 (merged)

mentioned in merge request !9842 (merged)

backported in !9842 (merged)