mp4: Remove ->error of struct mp4.

mp4: Remove ->error of struct mp4.

It's easier to have track_add(), the only function which sets ->error,
return an integer error code instead. Since track_add() is simple
and is only called by parse_sub_atoms(), open-code the logic there.

Also, don't reset ->total_tracks on errors because this leads to a
memory leak, don't increase the track counter on errors and remove
the comment which only states what is obvious.