[non-BE] Correct blending in blend_subfr2()

• Related issues: #1533 (closed)
• Requested reviewers: @gerstacker

Reason why this change is needed

• Wrong blending length in blend_subfr2() if called during transition to TCXonly modes

Description of the change

• Correct blending length in blend_subfr2()

Affected operating points

• Describe here as well as possible what operating points are affected and how
• In minimum, there should be a status for bitstream compatibility and output bit exactness
• For bitstream compatibility, the following levels are helpful for describing encoder
  1. Produced bitstream is BE compared to previous state.
  2. Produced bitstream is non-BE but it is fully backwards compatible for decoding. Decoded output may differ.
  3. Produced bitstream is non-BC. Old decoder cannot decode the produced bitstream correctly.
• For output difference, use BE or non-BE. Additionally, amount of difference can be presented.