net: ethernet: mtk_wed: fix possible NULL pointer dereference in mtk_wed_wo_queue_tx_...

net: ethernet: mtk_wed: fix possible NULL pointer dereference in mtk_wed_wo_queue_tx_clean()

In order to avoid a NULL pointer dereference, check entry->buf pointer before running
skb_free_frag in mtk_wed_wo_queue_tx_clean routine.

Fixes: 799684448e3e ("net: ethernet: mtk_wed: introduce wed wo support")
Signed-off-by: Lorenzo Bianconi <[email protected]>
Reviewed-by: Simon Horman <[email protected]>
Link: https://lore.kernel.org/r/3c1262464d215faa8acebfc08869798c81c96f4a.1702827359.git.lorenzo@kernel.org
Signed-off-by: Paolo Abeni <[email protected]>