Resolve "USAN: division-by-zero in levinson durbin algorithm in WB TBE"

• Related issues: #1130 (closed)
• Requested reviewers: @kiene

Reason why this change is needed

The autocorrelation coefficient R[0] may be zero in WB TBE when entering the Levinson-Durbin algorithm. This leads to the division-by-zero error reported in the USAN tests at the following line:

    rc[0] = ( -r[1] ) / r[0];

Description of the change

Prevent R[0] from being zero when entering the Levinson-Durbin algorithm in WB TBE by adding a small constant value.

Affected operating points

This is a non-BE fix affecting the core-coder. All operating points running with 16 kHz input signal may be affected. The fix ensures that EVS bit-exactness within the IVAS codec is not affected. However, the fix should be applied in the legacy EVS codec as well.