Fix T98345: Import von FBX didn't work.

Highly suspect the source FBX file to be broken, but investigating a 20MB
binary FBX file is not really an option right now, so cannot be 100%
sure. This can only be realistically investigated if we get a much
smaller reproducible case.

In the mean while, add similar check about indices validity for source
FBX data as we already have for destination Blender data arrays. This
allows to keep importing instead of 'crshing' the import process at
least.