xen-netfront: re-order error checks in xennet_get_responses()

Check the retrieved grant reference first; there's no point trying to
have xennet_move_rx_slot() move invalid data (and further defer
recognition of the issue, likely making diagnosis yet more difficult).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>