Update CDAudio.cs to fix PCECD Lemmings

fixes #665

I'm pretty sure this is correct. Lemmings is looking well past the end of the last track. I'm not sure if this is on purpose or not, but according to the CUE there is nothing out there, so we at least aren't missing anything.

BizHawk.Emulation.Cores/Sound/CDAudio.cs

@@ -71,6 +71,10 @@ namespace BizHawk.Emulation.Cores.Components
 		{
 			var track = Disc.Session1.SeekTrack(lba);
 			if (track == null) return;
+			// returning Leadout means no sound is playing
+			// and since we don't have any information about where leadout ends
+			// return in this case as well to avoid track.Number+1 below
+			if (track == Disc.Session1.LeadoutTrack) return;
 
 			PlayingTrack = track.Number;
 			StartLBA = lba;
@@ -242,4 +246,4 @@ namespace BizHawk.Emulation.Cores.Components
 				EnsureSector();
 		}
 	}
-}
+}