Resolve "USAN: OOB indexing in E_ACELP_corrmatrix_fx function"

• Related issues: #2274 (closed)
• Requested reviewers:

Reason why this change is needed

• Fix USAN OOB indexing complaint

Description of the change

• pointer decrease instead of array OOB indexing

Affected operating points

-None

Closes #2274 (closed)