Return the actual packet count from VisibleFrameCount() for audio tracks

The frame count for audio tracks is mostly meaningless as the API
doesn't expose audio frames, but it's the only way to check if a track
has been indexed.

src/core/track.cpp

@@ -187,7 +187,7 @@ int FFMS_Track::RealFrameNumber(int Frame) const {
 }
 
 int FFMS_Track::VisibleFrameCount() const {
-	return RealFrameNumbers.size();
+	return TT == FFMS_TYPE_AUDIO ? Frames.size() : RealFrameNumbers.size();
 }
 
 void FFMS_Track::MaybeReorderFrames() {