dpif-netdev: Fix memory leak in dpif_netdev_flow_put()

miniflow_destroy() needs to be called after using miniflow_init().
Otherwise, if the miniflow mallocs data, then a memory leak may
occur.

Found by inspection.

Signed-off-by: Ryan Wilson <wryan@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>