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 <lorenzo@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/3c1262464d215faa8acebfc08869798c81c96f4a.1702827359.git.lorenzo@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>