fix point energy not matching floating point during silences

Basic info

• Float reference:
  • Encoder (float): e15ad0ad
  • Decoder (float):
• Fixed point:
  • Encoder (fixed): b9942967
  • Decoder (fixed):

Bug description

As shown on the figure below, for some reason the noise level of the fixed point is really high compared to the floating point (red circles), BUT it can be almost ok (green circle), so it seems not to be a precision limitation.

The figure was created at 13.2kbps, but it seems to be there, at least in part, for all bitrate <= 32kbps

from top to bottom:

• spectrogram of the input
• spectrogram floating point
• spectrogram fixed point
• coding mode
• spectrum between 5.33 and 5.57s

Ways to reproduce

Box folder: ...\Box_EXTERNAL_IVAS_BASOP_VERIFICATION\issues\issue-1420

IVAS_cod -max_band WB -ism 1 NULL  13200  48 cp06a1s05.wav bit.flt
IVAS_dec EXT 48 bit.flt out.fltflt.wav