Resolve "[CLANG18] MSAN: use-of-uninitialized-value in lib_dec/dec_tcx_fx.c:4035:5"

• Related issues: #2353 (closed)
• Requested reviewers: @kiene

Reason why this change is needed

• Pointer prm_sqQ is uninitialized in case of bfi == 1

Description of the change

• Set pointer prm_sqQ to NULL, as done in floating-point code

Affected operating points

• All BE

Closes #2353 (closed)