sfc: pass valid pointers from efx_enqueue_unwind

The bytes_compl and pkts_compl pointers passed to efx_dequeue_buffers
cannot be NULL. Add a paranoid warning to check this condition and fix
the one case where they were NULL.

efx_enqueue_unwind() is called very rarely, during error handling.
Without this fix it would fail with a NULL pointer dereference in
efx_dequeue_buffer, with efx_enqueue_skb in the call stack.

Fixes: e9117e50 ("sfc: Firmware-Assisted TSO version 2")
Reported-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Bert Kenward <bkenward@solarflare.com>
Tested-by: Jarod Wilson <jarod@redhat.com>
Acked-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>