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/MixerBoard.cpp

@@ -342,10 +342,12 @@ WaveTrack *MixerTrackCluster::GetRight() const
 {
   // TODO: more-than-two-channels
    auto left = GetWave();
-   if (left)
-      return * ++ TrackList::Channels(left).begin();
-   else
-      return nullptr;
+   if (left) {
+      auto channels = TrackList::Channels(left);
+      if ( channels.size() > 1 )
+         return * ++ channels.first;
+   }
+   return nullptr;
 }
 
 #ifdef EXPERIMENTAL_MIDI_OUT

src/effects/SBSMSEffect.cpp

@@ -253,8 +253,10 @@ bool EffectSBSMS::Process()
             auto end = leftTrack->TimeToLongSamples(mCurT1);
 
             // TODO: more-than-two-channels
-            WaveTrack *rightTrack =
-               * ++ TrackList::Channels(leftTrack).begin();
+            auto channels = TrackList::Channels(leftTrack);
+            WaveTrack *rightTrack = (channels.size() > 1)
+               ? * ++ channels.first
+               : nullptr;
             if (rightTrack) {
                double t;
 

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

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;