Resolve "Crash in ISM rate switching with BINAURAL_ROOM_REVERB"

• Related issues: #779 (closed)
• Requested reviewers: @kiene

Reason why this change is needed

• Crash in ISM bitrate switching with BINAURAL_ROOM_REVERB.

Description of the change

• Corrects a bug in setting the st_ivas->hReverb handle to NULL.

Affected operating points

• Affects ISM bitrate switching with BINAURAL_ROOM_REVERB
• Bitstream is BE