Resolve "[CLANG18] MSAN: use-of-uninitialized-value in lib_dec/ivas_mct_dec_fx.c:225:13"

• Related issues: #2382 (closed)
• Requested reviewers: @kiene

Reason why this change is needed

Reading of uninitialized data when copying + scaling of Aq_fx[][][] to Aq_fx_32[][][]

Description of the change

Restrict operation to valid memory only. This is similar as in #2352 (closed)

Affected operating points

All operation points remain BE

Closes #2382 (closed)