Resolve "USAN: out-of-bounds-index in lib_dec/dec_acelp.c:175:19"

• Related issues: #1477 (closed)
• Requested reviewers:

Reason why this change is needed

• Fix OOB indexing in sanity check by moving the sanity check into the correct condition

Description of the change

• Move sanity check to correct place

Affected operating points

• tbc, probably none

Related to #1477 (closed)