bs1770demo can produce NaN as loudness result

The bs1770demo python wrapper function relies on the "Obtained level" and "Scaling factor" output by the bs1770demo binary: https://forge.3gpp.org/rep/ivas-codec-pc/ivas-processing-scripts/-/blob/main/ivas_processing_scripts/audiotools/wrappers/bs1770.py#L136-138

For certain files, this value can be reported as nan, an example is attached (found by @treffehn while working on background noise mixing):

tmp_loudness.pcm

❯ bs1770demo -nchan 2 tmp_loudness.pcm /dev/null
Input file:       tmp_loudness.pcm
Output file:      /dev/null
nchan:            2
Input level:      nan
Target level:     -26.000000
Obtained level:   nan
Scaling factor:   nan

--> Done processing 170667 samples

We need to decide what to do in such a scenario; I see the following options:

1. Patch bs1770demo to fix this (not sure how easy this is)
2. Use the RMS value computed in python
3. Use the RMS value reported from bs1770demo (I think there was a patch in the works?)