Resolve "ASAN: ACELP errors with 2 ISM coding using LTV"

• Related issues: #729 (closed)
• Requested reviewers:

Reason why this change is needed

• Observed ASAN and crash at the ISM encoder for 2 ISM at 64 kbps for specific MD values.

Description of the change

• If the bitrate for ACELP at 16kHz core coding is too low, the core decision is changed to TCX.

Affected operating points

• Affects 2ISM coding at 64kbps
• Bitstream is non-BE but it is BC.