[UI] corrected segfault when enabling shuffle on en empty play queue

src/ui/audio/PlayQueue.cpp

@@ -111,6 +111,7 @@ class TrackSelector
 TrackSelector::TrackSelector()
 : _loop(false),
 _shuffle(false),
+_size(0),
 _curPos(0)
 {
 }
@@ -152,11 +153,15 @@ TrackSelector::setShuffle(bool enable)
 {
 	_shuffle = enable;
 
+	if (_size != 0)
+	{
+		// Update the current index we are playing if we switch shuffle
 		if (_shuffle)
 			setPosByRowId(_curPos);
 		else
 			setPos(_trackPos[_curPos]);
 	}
+}
 
 int
 TrackSelector::next()