Bug2086: Mixer board behavior...

... and also fixed analogous mistakes in some effects (Vamp, SoundTouch, and
SBSMS effects), found by review of all uses of TrackList::Channels.

The first member of the pair of iterators returned by TrackList::Channels
does not necessarily deference to a null pointer when it is incremented to
the end of the range.

src/effects/vamp/VampEffect.cpp

@@ -372,7 +372,9 @@ bool VampEffect::Process()
 
       unsigned channels = 1;
 
-      const WaveTrack *right = *channelGroup.first++;
+      // channelGroup now contains all but the first channel
+      const WaveTrack *right =
+         channelGroup.size() ? *channelGroup.first++ : nullptr;
       if (right)
       {
          channels = 2;