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: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>