IVAS_rend ignores missing ISM metadata files

The IVAS_rend executable does not throw an error when an insufficient number of metadata file paths is provided.

Example:

./IVAS_rend \
  -i  ./scripts/testv/stvOSBA_1ISM_3OA16c.wav \
  -if ISM1SBA3 \
  -o  out.wav \
  -of BINAURAL

  # Metadata missing, e.g.
  # -im ./scripts/testv/stvOSBA_1ISM_3OA16c_ISM1.csv