nextTrackId not set when file has invalid nextTrackid

[L2501]

When a file has nextTrackid that is in use the it is not changed after adding a new track

mp4v2/src/mp4file.cpp

Lines 2977 to 2988 in 86b5aaf

	// we need to search for a track id
	for (trackId = 1; trackId <= 0xFFFF; trackId++) {
	try {
	(void)FindTrackIndex(trackId);
	// KEEP LOOKING, this trackId is in use
	}
	catch (Exception* x) {
	// OK, this trackId is not in use, proceed
	delete x;
	return trackId;
	}
	}