Resolve "MSAN: use-of-uninitialized-value in lib_dec/er_dec_tcx_fx.c:189:15"

• Related issues: #2473 (closed)
• Requested reviewers: @kiene

Reason why this change is needed

• st_fx->Q_syn_factor is not initialized in case of EVS.

Description of the change

• Make sure, st_fx->Q_syn_factor is properly initialized at least at during startup.

Affected operating points

• All operating modes expected to be BE.

Closes #2473 (closed)