Scaling of old_synthesis part for first good frame after frameloss missing in MCT

In the course of fixing #124 (closed), it was discovered that in MCT modes, the scaling of the old TCX synthesis in the first good frame after frameloss periods is missing. This can result in a "burst" at the end of a frameloss period and in clicks. The fix is simple (do the scaling also for MCT modes), but can cause bigger non-BE differences in the output, so I wanted to fix it separately and not combine it with the fix for #124 (closed).

See the effect here:

1. Baseline behaviour

2. Fixed behaviour