mt76: mt7915: fix potential NPE in TXS processing

If skb is null, then don't try to dereference it.
Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt7915/mac.c

@@ -1328,7 +1328,7 @@ mt7915_mac_add_txs_skb(struct mt7915_dev *dev, struct mt76_wcid *wcid, int pid,
 	mt76_tx_status_lock(mdev, &list);
 	skb = mt76_tx_status_skb_get(mdev, wcid, pid, &list);
 	if (!skb)
-		goto out;
+		goto out_no_skb;
 
 	txs = le32_to_cpu(txs_data[0]);
 
@@ -1416,6 +1416,8 @@ mt7915_mac_add_txs_skb(struct mt7915_dev *dev, struct mt76_wcid *wcid, int pid,
 
 out:
 	mt76_tx_status_skb_done(mdev, skb, &list);
+
+out_no_skb:
 	mt76_tx_status_unlock(mdev, &list);
 
 	return !!skb;