When creating DASH streams, the TrackNumber is externally prescribed
and not derived from the number of streams in the AVFormatContext, so
if the number of tracks for a file using an explicit TrackNumber was
more than one, the resulting file would be broken (it would be impossible
to tell to which track a Block belongs if different tracks share the
same TrackNumber). So disallow this.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>