[non-BE] Resolve "Switching artifact in MDCT -> DFT stereo switching"

• Related issues: #1203 (closed)
• Requested reviewers: @fotopoulou

Reason why this change is needed

• Severe artifacts during MDCT to DFT stereo switching when MDCT ITD is not used.

Description of the change

• Properly update the old input signal buffers hCPE->input_mem[]

Affected operating points

• Not BE for MDCT stereo to DFT stereo mode switching.
• BC.