[non-BE][allow-regression] Resolve "Potential porting bug in map_params_dirac_to_stereo"

• Related issues: #2522 (closed)
• Requested reviewers: @weckbecker

Reason why this change is needed

• Copy and paste error for the side_gain calculation in map_params_dirac_to_stereo()

Description of the change

• Correct the indexing.

Affected operating points

• Small differences in decoder output when decoding modes featuring DirAC to stereo.

Closes #2522 (closed)