Resolve "[CLANG18] MSAN: use-of-uninitialized-value in lib_dec/ivas_dirac_dec_fx.c:3226:9"

• Related issues: #2354 (closed)
• Requested reviewers: @weckbecker

Reason why this change is needed

• There is MSAN warning on uninit use of q_diffuseness. This is indeed not set properly always and can even vary it seems.

Description of the change

• Initialize q_diffuseness to Q30 when the overall structure is allocated. The value may then still change later on.

Affected operating points

• Can be BE but it is also possible that uninit value has caused some signal difference.

Closes #2354 (closed)