Resolve "IVAS_rend custom loudspeaker layouts do not work with MASA input"

• Related issues: #1326 (closed)
• Requested reviewers: @pihlajakuja

Reason why this change is needed

• MASA to Custom LS crashes with an error in IVAS_Rend_AddInput()

Description of the change

• Correctly compute the number of output channels from the custom LS data.

Affected operating points

• MASA external renderer custom LS output

Related to #1326 (closed)