qcow2: Avoid integer overflow in get_refcount (CVE-2014-0143)

This ensures that the checks catch all invalid cluster indexes
instead of returning the refcount of a wrong cluster.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
