Ensure the Player's listener is zero'd out as part of Player::Destroy().

Also cleaned up a couple small warnings.

src/core/audio/Crossfader.cpp

@@ -165,7 +165,7 @@ void Crossfader::ProcessMessage(IMessage &message) {
                     percent = (1.0f - percent);
                 }
 
-                float outputVolume = globalVolume * percent;
+                double outputVolume = globalVolume * percent;
 
 #if 0
                 std::string dir = (fade->direction == FadeIn) ? "in" : "out";

src/core/audio/Player.cpp

@@ -150,6 +150,7 @@ void Player::Destroy() {
             return; /* already terminated (or terminating) */
         }
 
+        this->listener = nullptr;
         this->state = Player::Quit;
         this->writeToOutputCondition.notify_all();
         this->thread->detach();

src/musikbox/app/window/TransportWindow.cpp

@@ -387,7 +387,7 @@ void TransportWindow::Update(TimeMode timeMode) {
         OFF(c, disabled);
     }
     else {
-        secondsTotal = transport.GetDuration();
+        secondsTotal = (int) transport.GetDuration();
 
         std::string title, album, artist;