Damn, the dirty rectangle values are signed integers.  So the checks
added by commit 788fbf04 are not good
enough, we also have to make sure they are not negative.

[ Note: There must be something broken in spice-server so we get
  negative values in the first place.  Bug opened:
  https://bugzilla.redhat.com/show_bug.cgi?id=1135372 ]

Cc: qemu-stable@nongnu.org
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Reviewed-by: NDr. David Alan Gilbert <dgilbert@redhat.com>