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/SoundTouchEffect.cpp

@@ -122,7 +122,10 @@ bool EffectSoundTouch::ProcessWithTimeWarper(const TimeWarper &warper)
 
             // TODO: more-than-two-channels
             auto channels = TrackList::Channels(leftTrack);
-            if (auto rightTrack = * ++ channels.begin()) {
+            auto rightTrack = (channels.size() > 1)
+               ? * ++ channels.first
+               : nullptr;
+            if ( rightTrack ) {
                double t;
 
                //Adjust bounds by the right tracks markers