With multiview/multilayer OpenEXR file, `load_image_single()` will return NULL ibuf,
since it has already populated ima (with `image_create_multiview()` or
`image_create_multilayer()` calls).
Also, added some more checks before doing `IMB_ImBufFromStereo3d()`, to be sure
we do have enough slots in ibuf_arr, and we do not overwrite second ibuf either.
ab85c5f980