parallels: Sanity check for s->tracks (CVE-2014-0142)

parallels: Sanity check for s->tracks (CVE-2014-0142)

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>
(cherry picked from commit 9302e863aa8baa5d932fc078967050c055fa1a7f)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>