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: 79968444 ("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.orgSigned-off-by: Paolo Abeni <pabeni@redhat.com>

drivers/net/ethernet/mediatek/mtk_wed_wo.c

@@ -291,6 +291,9 @@ mtk_wed_wo_queue_tx_clean(struct mtk_wed_wo *wo, struct mtk_wed_wo_queue *q)
 	for (i = 0; i < q->n_desc; i++) {
 		struct mtk_wed_wo_queue_entry *entry = &q->entry[i];
 
+		if (!entry->buf)
+			continue;
+
 		dma_unmap_single(wo->hw->dev, entry->addr, entry->len,
 				 DMA_TO_DEVICE);
 		skb_free_frag(entry->buf);