This avoids a possible division by zero.
Convert s->tracks to unsigned as well because it feels better than
surviving just because the results of calculations with s->tracks are
converted to unsigned anyway.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>