fw_cfg: fix memory corruption when all fw_cfg slots are used

When all the fw_cfg slots are used, a write is made outside the
bounds of the fw_cfg files array as part of the sort algorithm.

Fix it by avoiding an unnecessary array element move.
Fix also an assert while at it.

Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
Message-Id: <20180108215007.46471-1-marcel@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
