[non-BE] Fix glitch when stereo signal is decoded to mono

• Related issues: #1289 (closed)
• Requested reviewers: @multrus

Reason why this change is needed

• Fix glitch when the stereo signal is decoded to mono.

Description of the change

• Fix discontinuity in switching to DFT stereo in mono output.

Affected operating points

• Not BE for
• • Stereo to mono output when switching from TD to DFT stereo
• • When switching from SCE to DFT stereo for all outputs