Resolve "USAN: use-of-uninitialized-value in lib_dec/hf_synth_fx.c:1065:9"

• Related issues: #2459 (closed)
• Requested reviewers: @vaclav @vaillancourt

Reason why this change is needed

• Usan error in AMR-WB decoder when bitstream starts with an SID

Description of the change

• Initialize variable which is not used directly

Affected operating points

• None

Closes #2459 (closed)