Resolve "Uninitialized value used in stereo_dft_enc_itd_vad()"

• Related issues: #1152 (closed) sa4/audio/ivas-basop#828 (closed)
• Requested reviewers:

Reason why this change is needed

• Fix use of uninitialized value which is not problematic in float code, but causes problems in BASOP.

Description of the change

• change starting index of loop so that only initialized values are accessed
• array with result of computations is later only accessed starting from the same index

Affected operating points

• None, BE change
• BASOPs do not crash anymore

Related to #1152 (closed)